Hybrid beamforming

ABSTRACT

A system such as a speakerphone may include a processor, memory and an array of microphones. The processor may be configured (via program instructions stored in the memory) to perform automatic echo cancellation, self calibration and beam forming. In particular, the processor may receive input signals from the microphone array and operate on the input signals with a highly directed virtual beam which is a composite of two or more beams which are restricted to respective frequency ranges. The two or more beams may include beams of different kinds, e.g., superdirective beams and delay-and-sum beams.

PRIORITY CLAIMS

This application claims priority to U.S. Provisional Application No.60/676,415, filed on Apr. 29, 2005, entitled “SpeakerphoneFunctionality”, invented by William V. Oxford, Vijay Varadarajan andIoannis S. Dedes.

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/108,341, filed on Apr. 18, 2005, now U.S. Pat. No. 7,826,624entitled “Speakerphone Self Calibration and Beam Forming”, invented byWilliam V. Oxford and Vijay Varadarajan, which claims priority to U.S.Provisional Application No. 60/619,303 filed Oct. 15, 2004 and to U.S.Provisional Application No. 60/634,315 filed Dec. 8, 2004.

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/251,084, filed on Oct. 14, 2005, now U.S. Pat. No. 7,720,232entitled “Speakerphone”, invented by William V. Oxford, which claimspriority to U.S. Provisional Application No. 60/619,303 filed Oct. 15,2004 and to U.S. Provisional Application No. 60/634,315 filed Dec. 8,2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of communicationdevices and, more specifically, to speakerphones.

2. Description of the Related Art

Speakerphones are used in many types of telephone calls, andparticularly are used in conference calls where multiple people arelocated in a single room. A speakerphone may have one or moremicrophones to pick up voices of in-room participants, and at least onespeaker to audibly present voices from offsite participants. Whilespeakerphones may allow several people to participate in a conferencecall, there are a number of problems associated with the use ofspeakerphones.

As the microphone and speaker age, their physical properties change,thus compromising the ability to perform high quality acoustic echocancellation. Thus, there exists a need for a system and method capableof estimating descriptive parameters for the speaker and the microphoneas they age.

Furthermore, noise sources such as fans, electrical appliances and airconditioning interfere with the ability to discern the voices of theconference participants. Thus, there exists a need for a system andmethod capable of “tuning in” on the voices of the conferenceparticipants and “tuning out” the noise sources.

SUMMARY

In one set of embodiments, a system may include a set of microphones, amemory and a processor. The memory may be configured to store programinstructions. The processor may be configured to read and execute theprogram instructions from the memory. The program instructions areexecutable to implement:

-   -   (a) receiving input signals, one input signal corresponding to        each of the microphones;    -   (b) generating first versions of at least a first subset of the        input signals, wherein the first versions are band limited to a        first frequency range;    -   (c) operating on the first versions of the first subset of input        signals with a first set of parameters in order to compute a        first output signal corresponding to a first virtual beam having        an integer-order superdirective structure;    -   (d) generating second versions of at least a second subset of        the input signals, wherein the second versions are band limited        to a second frequency range different from the first frequency        range;    -   (e) operating on the second versions of the second subset of        input signals with a second set of parameters in order to        compute a second output signal corresponding to a second virtual        beam;    -   (f) generating a resultant signal, wherein the resultant signal        includes a combination of at least the first output signal and        the second output signal.        The second virtual beam may be a beam having a delay-and-sum        structure or an integer order superdirective structure, e.g.,        with integer order different from the integer order of the first        virtual beam.

The first subset of the input signals may correspond to a first subsetof the microphones which are at least partially aligned in a targetdirection. Furthermore, the second set of parameters may be configuredso as to direct the second virtual beam in the target direction.

Additional integer-order superdirective beams and/or delay-and-sum beamsmay be applied to corresponding subsets of band-limited versions of theinput signals, and the corresponding outputs (from the additional beams)may be combined into the resultant signal.

It is noted that (b) through (f) may be performed in the time domain, inthe frequency domain, or partly in the time domain and partly in thefrequency domain.

The program instructions may be further configured to direct theprocessor to provide the resultant signal to a communication interfacefor transmission to one or more remote devices.

The set of microphones may be arranged on a circle, ellipse, square orrectangle, or on a grid such as rectangular grid, hexagonal grid, etc.

In another set of embodiments, a method for generating a highly directedbeam may involve:

-   -   receiving input signals from an array of microphones, one input        signal from each of the microphones;    -   operating on the input signals with a set of virtual beams to        obtain respective beam-formed signals, wherein each of the        virtual beams is associated with a corresponding frequency range        and a corresponding subset of the input signals, wherein each of        the virtual beams operates on versions of the input signals of        the corresponding subset of input signals, wherein said versions        are band limited to the corresponding frequency range, wherein        the virtual beams include one or more virtual beams of a first        type and one or more virtual beams of a second type; and    -   generating a resultant signal, wherein the resultant signal        includes a combination of the beam-formed signals.        In some embodiments, the one or more beams of the first type may        be integer-order superdirective beams (although, not necessarily        all of the same integer order), and the one or more beams of the        second type may be delay-and-sum beams.

In one embodiment, the method may further involve: generating firstversions of at least a first subset of the input signals, wherein thefirst versions are band-limited to a first frequency range; andgenerating second versions of at least a second subset of the inputsignals, wherein the second versions are band-limited to a secondfrequency range different from the first frequency range. Furthermore,the action of operating on the input signals with a set of virtual beamsto obtain respective beam-formed signals may include:

-   -   operating on the first versions of the first subset of input        signals with a first set of parameters in order to compute a        first beam-formed signal corresponding to a first of the one or        more virtual beams of the first type;    -   operating on the second versions of the second subset of input        signals with a second set of parameters in order to compute a        second beam-formed signal corresponding to a first of the one or        more virtual beams of the second type.        The first subset of the input signals may correspond to a first        subset of the microphones which are at least partially aligned        in a target direction. In addition, the second set of parameters        may be configured so as to direct the first virtual beam of the        second type in the target direction.

In one embodiment, the first virtual beam of the first type is aninteger-order superdirective beam, and the first virtual beam of thesecond type is either a beam having a delay-and-sum structure, or, aninteger-order superdirective beam with a different integer order thanthe first virtual beam of the first type.

The action of operating on the input signals with a set of virtual beamsto obtain respective beam-formed signals may be performed in thetime-domain, in the frequency domain, or, partly in each domain.

The resultant signal may be provided to a communication interface fortransmission to one or more remote destinations, e.g., one or more otherspeakerphones.

The array of microphones may be arranged on a circle. In this case, thesecond set of parameters may be generated by accessing an array ofparameters from memory, and applying a circular shift to the array ofparameters, where an amount of the shift corresponds to a desired targetdirection.

In yet another set of embodiments, a computer readable memory medium maybe configured to store program instructions, where the programinstructions are executable to implement:

-   -   (a) receiving input signals from an array of microphones, one        input signal corresponding to each of the microphones;    -   (b) generating first versions of at least a first subset of the        input signals, wherein the first versions are band-limited to a        first frequency range;    -   (c) operating on the first versions of the first subset of input        signals with a first set of parameters in order to compute a        first output signal corresponding to a first virtual beam having        an integer-order superdirective structure;    -   (d) generating second versions of at least a second subset of        the input signals, wherein the second versions are band limited        to a second frequency range different from the first frequency        range;    -   (e) operating on the second versions of the second subset of        input signals with a second set of parameters in order to        compute a second output signal corresponding to a second virtual        beam;    -   (f) generating a resultant signal, wherein the resultant signal        includes a combination of at least the first output signal and        the second output signal.

In yet another set of embodiments, a method for configuring a system(i.e., a system including an array of microphones, a memory and aprocessor) may involve:

-   -   (a) generating a first set of parameters for a first virtual        beam based on a first subset of the microphones, where the first        virtual beam has an integer-order superdirective structure;    -   (b) computing a plurality of parameter sets for a corresponding        plurality of delay-and-sum beams, where the parameter set for        each delay-and-sum beam is computed for a corresponding        frequency, where the parameter sets for the delay-and-sum beams        are computed based on a common set of beam constraints, where        the frequencies for the delay-and-sum beams are above a        transition frequency;    -   (c) combining the plurality of parameter sets to obtain a second        set of parameters;    -   (d) storing the first set of parameters and the second set of        parameters in the memory of the system.

The system may be a device such as a speakerphone, a videoconferencingsystem, a surveillance device, a video camera, etc. The common set ofbeam constraints may include specification of a common pass-band width(i.e., main lobe width).

In yet another set of embodiments, a method for configuring a system(i.e., a system including an array of microphones, a memory and aprocessor) may involve:

-   -   (a) generating a first set of parameters for a first virtual        beam based on a first subset of the microphones, where the first        virtual beam has an integer-order superdirective structure;    -   (b) computing a plurality of parameter sets for a corresponding        plurality of delay-and-sum beams, where the parameter set for        each delay-and-sum beam is computed for a corresponding        frequency, where the parameter sets for the delay-and-sum beams        are computed based on a common set of beam constraints, where        the frequencies for the delay-and-sum beams are above a        transition frequency;    -   (c) storing the first set of parameters and the plurality of        parameter sets in the memory of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description makes reference to the accompanyingdrawings, which are now briefly described.

FIG. 1A illustrates communication system including two speakerphonescoupled through a communication mechanism.

FIG. 1B illustrates one set of embodiments of a speakerphone system 200.

FIG. 2 illustrates a direct path transmission and three examples ofreflected path transmissions between the speaker 255 and microphone 201.

FIG. 3 illustrates a diaphragm of an electret microphone.

FIG. 4A illustrates the change over time of a microphone transferfunction.

FIG. 4B illustrates the change over time of the overall transferfunction due to changes in the properties of the speaker over time underthe assumption of an ideal microphone.

FIG. 5 illustrates a lowpass weighting function L(ω).

FIG. 6A illustrates one set of embodiments of a method for performingoffline self calibration.

FIG. 6B illustrates one set of embodiments of a method for performing“live” self calibration.

FIG. 7 illustrates one embodiment of speakerphone having a circulararray of microphones.

FIG. 8 illustrates one set of embodiments of a speakerphone 300configured to cancel a direct path signal from to inputpreamplification.

FIG. 8B illustrates one embodiments of the speakerphone 300 having anEthernet bridge.

FIG. 9 illustrates one embodiment of a software block diagram that maybe executed by processor 207.

FIG. 9B illustrates one embodiment of a method for canceling speakersignal energy from a received microphone signal.

FIG. 10 illustrates one embodiment of speakerphone 300 configured toperform a separate direct path cancellation on each microphone inputchannel.

FIG. 10B illustrates one embodiment of speakerphone 300 configured togenerate a single cancellation signal which is applied to all microphoneinput channels.

FIG. 11 illustrates circuitry to shift the phases of an A/D conversionclock and a D/A conversion clock relative to a base conversion clock.

FIG. 12 illustrates an example of design parameters associated with thedesign of a beam B(i).

FIG. 13 illustrates two sets of three microphones aligned approximatelyin a target direction, each set being used to form a virtual beam.

FIG. 14 illustrates three sets of two microphones aligned in a targetdirection, each set being used to form a virtual beam.

FIG. 15 illustrates two sets of four microphones aligned in a targetdirection, each set being used to form a virtual beam.

FIG. 16A illustrates one set of embodiments of a method for forming ahighly directed beam using at least an integer-order superdirective beamand a delay-and-sum beam;

FIG. 16B illustrates one set of embodiments of a method for forming ahighly directed beam using at least a first virtual beam and a secondvirtual beam in different frequency ranges;

FIG. 16C illustrates one set of embodiments of a method for forming ahighly directed beam using one or more virtual beams of a first type andone or more virtual beams of a second type;

FIG. 17 illustrates one set of embodiments of a method for configured asystem having an array of microphones, a processor and a method.

FIG. 18 illustrates one embodiment of a microphone having a diaphragm303.

FIG. 19 illustrates one set of embodiments of a method for offsettingmicrophone drift.

FIG. 20 illustrates a virtual linear array derived from a physicalcircular array of microphones.

FIG. 21A illustrates a broadside linear array.

FIG. 21B illustrates an endfire linear array.

FIG. 21C illustrates a non-uniformly space endfire array.

FIG. 21D illustrates the sensitivity pattern of a highly directedvirtual microphone.

FIG. 21E illustrates one set of embodiments of a method for generating ahighly directed virtual microphone pointed at an acoustic source using auniform circular array of physical microphones.

While the invention is described herein by way of example for severalembodiments and illustrative drawings, those skilled in the art willrecognize that the invention is not limited to the embodiments ordrawings described. It should be understood, that the drawings anddetailed description thereto are not intended to limit the invention tothe particular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims. The headings used herein are for organizational purposes onlyand are not meant to be used to limit the scope of the description orthe claims. As used throughout this application, the word “may” is usedin a permissive sense (i.e., meaning having the potential to), ratherthan the mandatory sense (i.e., meaning must). Similarly, the words“include”, “including”, and “includes” mean including, but not limitedto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

U.S. Provisional Application No. 60/676,415, filed on Apr. 29, 2005,entitled “Speakerphone Functionality”, invented by William V. Oxford,Vijay Varadarajan and Ioannis S. Dedes, is hereby incorporated byreference in its entirety.

U.S. Patent Application Oct. 14, 2005, filed on Oct. 14, 2005, entitled“Speakerphone”; invented by William V. Oxford, is hereby incorporated byreference in its entirety.

U.S. patent application Ser. No. 11/108,341, filed on Apr. 18, 2005,entitled “Speakerphone Self Calibration and Beam Forming”, invented byWilliam V. Oxford and Vijay Varadarajan, is hereby incorporated byreference in its entirety.

U.S. Provisional Patent Application titled “Video ConferencingSpeakerphone”, Ser. No. 60/619,212, which was filed Oct. 15, 2004, whoseinventors are Michael L. Kenoyer, Craig B. Malloy, and Wayne E. Mock ishereby incorporated by reference in its entirety.

U.S. Provisional Patent Application titled “Video Conference CallSystem”, Ser. No. 60/619,210, which was filed Oct. 15, 2004, whoseinventors are Michael J. Burkett, Ashish Goyal, Michael V. Jenkins,Michael L. Kenoyer, Craig B. Malloy, and Jonathan W. Tracey is herebyincorporated by reference in its entirety.

U.S. Provisional Patent Application titled “High Definition Camera andMount”, Ser. No. 60/619,227, which was filed Oct. 15, 2004, whoseinventors are Michael L. Kenoyer, Patrick D. Vanderwilt, Paul D. Frey,Paul Leslie Howard, Jonathan I. Kaplan, and Branko Lukic, is herebyincorporated by reference in its entirety.

LIST OF ACRONYMS USED HEREIN

-   DDR SDRAM=Double-Data-Rate Synchronous Dynamic RAM-   DRAM=Dynamic RAM-   FIFO=First-In First-Out Buffer-   FIR=Finite Impulse Response-   FFT=Fast Fourier Transform-   Hz=Hertz-   IIR=Infinite Impulse Response-   ISDN=Integrated Services Digital Network-   kHz=kiloHertz-   PSTN=Public Switched Telephone Network-   RAM=Random Access Memory-   RDRAM=Rambus Dynamic RAM-   ROM=Read Only Memory-   SDRAM=Synchronous Dynamic Random Access Memory-   SRAM=Static RAM

A communication system may be configured to facilitate voicecommunication between participants (or groups of participants) who arephysically separated as suggested by FIG. 1A. The communication systemmay include a first speakerphone SP₁ and a second speakerphone SP₂coupled through a communication mechanism CM. The communicationmechanism CM may be realized by any of a wide variety of well knowncommunication technologies. For example, communication mechanism CM maybe the PSTN (public switched telephone network) or a computer networksuch as the Internet.

Speakerphone Block Diagram

FIG. 1B illustrates a speakerphone 200 according to one set ofembodiments. The speakerphone 200 may include a processor 207 (or a setof processors), memory 209, a set 211 of one or more communicationinterfaces, an input subsystem and an output subsystem.

The processor 207 is configured to read program instructions which havebeen stored in memory 209 and to execute the program instructions inorder to enact any of the various methods described herein.

Memory 209 may include any of various kinds of semiconductor memory orcombinations thereof. For example, in one embodiment, memory 209 mayinclude a combination of Flash ROM and DDR SDRAM.

The input subsystem may include a microphone 201 (e.g. an electretmicrophone), a microphone preamplifier 203 and an analog-to-digital(A/D) converter 205. The microphone 201 receives an acoustic signal A(t)from the environment and converts the acoustic signal into an electricalsignal u(t). (The variable t denotes time.) The microphone preamplifier203 amplifies the electrical signal u(t) to produce an amplified signalx(t). The A/D converter samples the amplified signal x(t) to generatedigital input signal X(k). The digital input signal X(k) is provided toprocessor 207.

In some embodiments, the A/D converter may be configured to sample theamplified signal x(t) at least at the Nyquist rate for speech signals.In other embodiments, the A/D converter may be configured to sample theamplified signal x(t) at least at the Nyquist rate for audio signals.

Processor 207 may operate on the digital input signal X(k) to removevarious sources of noise, and thus, generate a corrected microphonesignal Z(k). The processor 207 may send the corrected microphone signalZ(k) to one or more remote devices (e.g., a remote speakerphone) throughone or more of the set 211 of communication interfaces.

The set 211 of communication interfaces may include a number ofinterfaces for communicating with other devices (e.g., computers orother speakerphones) through well-known communication media. Forexample, in various embodiments, the set 211 includes a networkinterface (e.g., an Ethernet bridge), an ISDN interface, a PSTNinterface, or, any combination of these interfaces.

The speakerphone 200 may be configured to communicate with otherspeakerphones over a network (e.g., an Internet Protocol based network)using the network interface. In one embodiment, the speakerphone 200 isconfigured so multiple speakerphones, including speakerphone 200, may becoupled together in a daisy chain configuration.

The output subsystem may include a digital-to-analog (D/A) converter240, a power amplifier 250 and a speaker 225. The processor 207 mayprovide a digital output signal Y(k) to the D/A converter 240. The D/Aconverter 240 converts the digital output signal Y(k) to an analogsignal y(t). The power amplifier 250 amplifies the analog signal y(t) togenerate an amplified signal v(t). The amplified signal v(t) drives thespeaker 225. The speaker 225 generates an acoustic output signal inresponse to the amplified signal v(t).

Processor 207 may receive a remote audio signal R(k) from a remotespeakerphone through one of the communication interfaces and mix theremote audio signal R(k) with any locally generated signals (e.g., beepsor tones) in order to generate the digital output signal Y(k). Thus, theacoustic signal radiated by speaker 225 may be a replica of the acousticsignals (e.g., voice signals) produced by remote conference participantssituated near the remote speakerphone.

In one alternative embodiment, the speakerphone may include circuitryexternal to the processor 207 to perform the mixing of the remote audiosignal R(k) with any locally generated signals.

In general, the digital input signal X(k) represents a superposition ofcontributions due to:

-   -   acoustic signals (e.g., voice signals) generated by one or more        persons (e.g., conference participants) in the environment of        the speakerphone 200, and reflections of these acoustic signals        off of acoustically reflective surfaces in the environment;    -   acoustic signals generated by one or more noise sources (such as        fans and motors, automobile traffic and fluorescent light        fixtures) and reflections of these acoustic signals off of        acoustically reflective surfaces in the environment; and    -   the acoustic signal generated by the speaker 225 and the        reflections of this acoustic signal off of acoustically        reflective surfaces in the environment.

Processor 207 may be configured to execute software including anacoustic echo cancellation (AEC) module. The AEC module attempts toestimate the sum C(k) of the contributions to the digital input signalX(k) due to the acoustic signal generated by the speaker and a number ofits reflections, and, to subtract this sum C(k) from the digital inputsignal X(k) so that the corrected microphone signal Z(k) may be a higherquality representation of the acoustic signals generated by the localconference participants.

In one set of embodiments, the AEC module may be configured to performmany (or all) of its operations in the frequency domain instead of inthe time domain. Thus, the AEC module may:

-   -   estimate the Fourier spectrum C(ω) of the signal C(k) instead of        the signal C(k) itself, and    -   subtract the spectrum C(ω) from the spectrum X(ω) of the input        signal X(k) in order to obtain a spectrum Z(ω).        An inverse Fourier transform may be performed on the spectrum        Z(ω) to obtain the corrected microphone signal Z(k). As used        herein, the “spectrum” of a signal is the Fourier transform        (e.g., the FFT) of the signal.

In order to estimate the spectrum C(ω), the acoustic echo cancellationmodule may utilize:

-   -   the spectrum Y(ω) of a set of samples of the output signal Y(k),        and modeling information I_(M) describing the input-output        behavior of the system elements (or combinations of system        elements) between the circuit nodes corresponding to signals        Y(k) and X(k).

For example, in one set of embodiments, the modeling information I_(M)may include:

-   -   (a) a gain of the D/A converter 240;    -   (b) a gain of the power amplifier 250;    -   (c) an input-output model for the speaker 225;    -   (d) parameters characterizing a transfer function for the direct        path and reflected path transmissions between the output of        speaker 225 and the input of microphone 201;    -   (e) a transfer function of the microphone 201;    -   (f) a gain of the preamplifier 203;    -   (g) a gain of the A/D converter 205.        The parameters (d) may include attenuation coefficients and        propagation delay times for the direct path transmission and a        set of the reflected path transmissions between the output of        speaker 225 and the input of microphone 201. FIG. 2 illustrates        the direct path transmission and three reflected path        transmission examples.

In some embodiments, the input-output model for the speaker may be (ormay include) a nonlinear Volterra series model, e.g., a Volterra seriesmodel of the form:

$\begin{matrix}{{{f_{S}(k)} = {{\sum\limits_{i = 0}^{N_{a} - 1}{a_{i}{v\left( {k - i} \right)}}} + {\sum\limits_{i = 0}^{N_{b} - 1}{\sum\limits_{j = 0}^{M_{b} - 1}{b_{ij}{{v\left( {k - i} \right)} \cdot {v\left( {k - j} \right)}}}}}}},} & (1)\end{matrix}$where v(k) represents a discrete-time version of the speaker's inputsignal, where f_(S)(k) represents a discrete-time version of thespeaker's acoustic output signal, where N_(a), N_(b) and M_(b) arepositive integers. For example, in one embodiment, N_(a)=8, N_(b)=3 andM_(b)=2. Expression (1) has the form of a quadratic polynomial. Otherembodiments using higher order polynomials are contemplated.

In alternative embodiments, the input-output model for the speaker is atransfer function (or equivalently, an impulse response).

In one embodiment, the AEC module may compute the compensation spectrumC(ω) using the output spectrum Y(ω) and the modeling information I_(M)(including previously estimated values of the parameters (d)).Furthermore, the AEC module may compute an update for the parameters (d)using the output spectrum Y(ω), the input spectrum X(ω), and at least asubset of the modeling information I_(M) (possibly including thepreviously estimated values of the parameters (d)).

In another embodiment, the AEC module may update the parameters (d)before computing the compensation spectrum C(ω).

In those embodiments where the speaker input-output model is a nonlinearmodel (such as a Volterra series model), the AEC module may be able toconverge more quickly and/or achieve greater accuracy in its estimationof the attenuation coefficients and delay times (of the direct path andreflected paths) because it will have access to a more accuraterepresentation of the actual acoustic output of the speaker than inthose embodiments where a linear model (e.g., a transfer function) isused to model the speaker.

In some embodiments, the AEC module may employ one or more computationalalgorithms that are well known in the field of echo cancellation.

The modeling information I_(M) (or certain portions of the modelinginformation I_(M)) may be initially determined by measurements performedat a testing facility prior to sale or distribution of the speakerphone200. Furthermore, certain portions of the modeling information I_(M)(e.g., those portions that are likely to change over time) may berepeatedly updated based on operations performed during the lifetime ofthe speakerphone 200.

In one embodiment, an update to the modeling information I_(M) may bebased on samples of the input signal X(k) and samples of the outputsignal Y(k) captured during periods of time when the speakerphone is notbeing used to conduct a conversation.

In another embodiment, an update to the modeling information I_(M) maybe based on samples of the input signal X(k) and samples of the outputsignal Y(k) captured while the speakerphone 200 is being used to conducta conversation.

In yet another embodiment, both kinds of updates to the modelinginformation I_(M) may be performed.

Updating Modeling Information Based on Offline Calibration Experiments

In one set of embodiments, the processor 207 may be programmed to updatethe modeling information I_(M) during a period of time when thespeakerphone 200 is not being used to conduct a conversation.

The processor 207 may wait for a period of relative silence in theacoustic environment. For example, if the average power in the inputsignal X(k) stays below a certain threshold for a certain minimum amountof time, the processor 207 may reckon that the acoustic environment issufficiently silent for a calibration experiment. The calibrationexperiment may be performed as follows.

The processor 207 may output a known noise signal as the digital outputsignal Y(k). In some embodiments, the noise signal may be a burst ofmaximum-length-sequence noise, followed by a period of silence. Forexample, in one embodiment, the noise signal burst may be approximately2-2.5 seconds long and the following silence period may be approximately5 seconds long. In some embodiments, the noise signal may be submittedto one or more notch filters (e.g., sharp notch filters), in order tonull out one or more frequencies known to causes resonances ofstructures in the speakerphone, prior to transmission from the speaker.

The processor 207 may capture a block B_(X) of samples of the digitalinput signal X(k) in response to the noise signal transmission. Theblock B_(X) may be sufficiently large to capture the response to thenoise signal and a sufficient number of its reflections for a maximumexpected room size.

The block B_(X) of samples may be stored into a temporary buffer, e.g.,a buffer which has been allocated in memory 209.

The processor 207 computes a Fast Fourier Transform (FFT) of thecaptured block B_(X) of input signal samples X(k) and an FFT of acorresponding block B_(Y) of samples of the known noise signal Y(k), andcomputes an overall transfer function H(ω) for the current experimentaccording to the relationH(ω)=FFT(B _(X))/FFT(B _(Y)),  (2)where ω denotes angular frequency. The processor may make specialprovisions to avoid division by zero.

The processor 207 may operate on the overall transfer function H(ω) toobtain a midrange sensitivity value s₁ as follows.

The midrange sensitivity value s₁ may be determined by computing anA-weighted average of the magnitude of the overall transfer functionH(ω):s ₁=SUM[|H(ω)|A(ω),ω ranging from zero to π].  (3)

In some embodiments, the weighting function A(ω) may be designed so asto have low amplitudes:

-   -   at low frequencies where changes in the overall transfer        function due to changes in the properties of the speaker are        likely to be expressed, and    -   at high frequencies where changes in the overall transfer        function due to material accumulation on the microphone        diaphragm are likely to be expressed.

The diaphragm of an electret microphone is made of a flexible andelectrically non-conductive material such as plastic (e.g., Mylar) assuggested in FIG. 3. Charge (e.g., positive charge) is deposited on oneside of the diaphragm at the time of manufacture. A layer of metal maybe deposited on the other side of the diaphragm.

As the microphone ages, the deposited charge slowly dissipates,resulting in a gradual loss of sensitivity over all frequencies.Furthermore, as the microphone ages material such as dust and smokeaccumulates on the diaphragm, making it gradually less sensitive at highfrequencies. The summation of the two effects implies that the amplitudeof the microphone transfer function |H_(mic)(ω)| decreases at allfrequencies, but decreases faster at high frequencies as suggested byFIG. 4A. If the speaker were ideal (i.e., did not change its propertiesover time), the overall transfer function H(ω) would manifest the samekind of changes over time.

The speaker 225 includes a cone and a surround coupling the cone to aframe. The surround is made of a flexible material such as butyl rubber.As the surround ages it becomes more compliant, and thus, the speakermakes larger excursions from its quiescent position in response to thesame current stimulus. This effect is more pronounced at lowerfrequencies and negligible at high frequencies. In addition, the longerexcursions at low frequencies implies that the vibrational mechanism ofthe speaker is driven further into the nonlinear regime. Thus, if themicrophone were ideal (i.e., did not change its properties over time),the amplitude of the overall transfer function H(ω) in expression (2)would increase at low frequencies and remain stable at high frequencies,as suggested by FIG. 4B.

The actual change to the overall transfer function H(ω) over time is dueto a combination of affects including the speaker aging mechanism andthe microphone aging mechanism just described.

In addition to the sensitivity value s₁, the processor 207 may compute alowpass sensitivity value s₂ and a speaker related sensitivity s₃ asfollows. The lowpass sensitivity factor s₂ may be determined bycomputing a lowpass weighted average of the magnitude of the overalltransfer function H(ω):s ₂=SUM[|H(ω)|L(ω),ω ranging from zero to π].  (4)

The lowpass weighting function L(ω) equals is equal (or approximatelyequal) to one at low frequencies and transitions towards zero in theneighborhood of a cutoff frequency. In one embodiment, the lowpassweighting function may smoothly transition to zero as suggested in FIG.5.

The processor 207 may compute the speaker-related sensitivity value s₃according to the expression:s ₃ =s ₂ −s ₁.

The processor 207 may maintain sensitivity averages S₁, S₂ and S₃corresponding to the sensitivity values s₁, s₂ and s₃ respectively. Theaverage S_(i), i=1, 2, 3, represents the average of the sensitivityvalue s_(i) from past performances of the calibration experiment.

Furthermore, processor 207 may maintain averages A_(i) and B_(ij)corresponding respectively to the coefficients a_(i) and b_(ij) in theVolterra series speaker model. After computing sensitivity value s₃, theprocessor may compute current estimates for the coefficients b_(ij) byperforming an iterative search. Any of a wide variety of known searchalgorithms may be used to perform this iterative search.

In each iteration of the search, the processor may select values for thecoefficients b_(ij) and then compute an estimated input signalX_(EST)(k) based on:

-   -   the block B_(Y) of samples of the transmitted noise signal Y(k);    -   the gain of the D/A converter 240 and the gain of the power        amplifier 250;    -   the modified Volterra series expression

$\begin{matrix}{{{f_{S}(k)} = {{c{\sum\limits_{i = 0}^{N_{a} - 1}{A_{i}{v\left( {k - 1} \right)}}}} + {\sum\limits_{i = 0}^{N_{b} - 1}{\sum\limits_{j = 0}^{M_{b} - 1}{b_{ij}{{v\left( {k - i} \right)} \cdot {v\left( {k - j} \right)}}}}}}},} & (5)\end{matrix}$

-   -   where c is given by c=s₃/S₃;    -   the parameters characterizing the transfer function for the        direct path and reflected path transmissions between the output        of speaker 225 and the input of microphone 201;    -   the transfer function of the microphone 201;    -   the gain of the preamplifier 203; and    -   the gain of the A/D converter 205.

The processor may compute the energy of the difference between theestimated input signal X_(EST)(k) and the block B_(X) of actuallyreceived input samples X(k). If the energy value is sufficiently small,the iterative search may terminate. If the energy value is notsufficiently small, the processor may select a new set of values for thecoefficients b_(ij), e.g., using knowledge of the energy values computedin the current iteration and one or more previous iterations.

The scaling of the linear terms in the modified Volterra seriesexpression (5) by factor c serves to increase the probability ofsuccessful convergence of the b_(ij).

After having obtained final values for the coefficients b_(ij), theprocessor 207 may update the average values B_(ij) according to therelations:B _(ij) ←k _(ij) B _(ij)+(1−k _(ij))b _(ij),  (6)where the values k_(ij) are positive constants between zero and one.

In one embodiment, the processor 207 may update the averages A_(i)according to the relations:A _(i) ←g _(i) A _(i)+(1−g _(i))(cA _(i)),  (7)where the values g_(i) are positive constants between zero and one.

In an alternative embodiment, the processor may compute currentestimates for the Volterra series coefficients a_(i) based on anotheriterative search, this time using the Volterra expression:

$\begin{matrix}{{f_{S}(k)} = {{\sum\limits_{i = 0}^{N_{a} - 1}{a_{i}{v\left( {k - i} \right)}}} + {\sum\limits_{i = 0}^{N_{b} - 1}{\sum\limits_{j = 0}^{M_{b} - 1}{B_{ij}{{v\left( {k - i} \right)} \cdot {{v\left( {k - j} \right)}.}}}}}}} & \left( {8A} \right)\end{matrix}$

After having obtained final values for the coefficients a_(i), theprocessor may update the averages A_(i) according the relations:A _(i) ←g _(i) A _(i)+(1−g _(i))a _(i).  (8B)

The processor may then compute a current estimate T_(mic) of themicrophone transfer function based on an iterative search, this timeusing the Volterra expression:

$\begin{matrix}{{f_{S}(k)} = {{\sum\limits_{i = 0}^{N_{a} - 1}{A_{i}{v\left( {k - i} \right)}}} + {\sum\limits_{i = 0}^{N_{b} - 1}{\sum\limits_{j = 0}^{M_{b} - 1}{B_{ij}{{v\left( {k - i} \right)} \cdot {{v\left( {k - j} \right)}.}}}}}}} & (9)\end{matrix}$

After having obtained a current estimate T_(mic) for the microphonetransfer function, the processor may update an average microphonetransfer function H_(mic) based on the relation:H _(mic)(ω)←k _(m) H _(mic)(ω)+(1−k _(m))T _(mic)(ω),  (10)where k_(m) is a positive constant between zero and one.

Furthermore, the processor may update the average sensitivity values S₁,S₂ and S₃ based respectively on the currently computed sensitivities s₁,s₂, s₃, according to the relations:S ₁ ←h ₁ S ₁+(1−h ₁)s ₁,  (11)S ₂ ←h ₂ S ₂+(1−h ₂)s ₂,  (12)S ₃ ←h ₃ S ₃+(1−h ₃)s ₃,  (13)where h₁, h₂, h₃ are positive constants between zero and one.

In the discussion above, the average sensitivity values, the Volterracoefficient averages A_(i) and B_(ij) and the average microphonetransfer function H_(mic) are each updated according to an IIR filteringscheme. However, other filtering schemes are contemplated such as FIRfiltering (at the expense of storing more past history data), variouskinds of nonlinear filtering, etc.

In one set of embodiments, a system (e.g., a speakerphone or avideoconferencing system) may include a microphone, a speaker, memoryand a processor, e.g., as illustrated in FIG. 1B. The memory may beconfigured to store program instructions and data. The processor isconfigured to read and execute the program instructions from the memory.The program instructions are executable by the processor to:

-   -   (a) output a stimulus signal (e.g., a noise signal) for        transmission from the speaker;    -   (b) receive an input signal from the microphone, corresponding        to the stimulus signal and its reverb tail;    -   (c) compute a midrange sensitivity and a lowpass sensitivity for        a spectrum of a transfer function H(ω) derived from a spectrum        of the input signal and a spectrum of the stimulus signal;    -   (d) subtract the midrange sensitivity from the lowpass        sensitivity to obtain a speaker-related sensitivity;    -   (e) perform an iterative search for current values of parameters        of an input-output model for the speaker using the input signal        spectrum, the stimulus signal spectrum, the speaker-related        sensitivity; and    -   (f) update averages of the parameters of the speaker        input-output model using the current values obtained in (e).        The parameter averages of the speaker input-output model are        usable to perform echo cancellation on other input signals.

The input-output model of the speaker may be a nonlinear model, e.g., aVolterra series model.

Furthermore, in some embodiments, the program instructions may beexecutable by the processor to:

-   -   perform an iterative search for a current transfer function of        the microphone using the input signal spectrum, the stimulus        signal spectrum, and the current values; and    -   update an average microphone transfer function using the current        transfer function.        The average transfer function is also usable to perform said        echo cancellation on said other input signals.

In another set of embodiments, as illustrated in FIG. 6A, a method forperforming self calibration may involve the following steps:

-   -   (a) outputting a stimulus signal (e.g., a noise signal) for        transmission from a speaker (as indicated at step 610);    -   (b) receiving an input signal from a microphone, corresponding        to the stimulus signal and its reverb tail (as indicated at step        615);    -   (c) computing a midrange sensitivity and a lowpass sensitivity        for a transfer function H(ω) derived from a spectrum of the        input signal and a spectrum of the stimulus signal (as indicated        at step 620);    -   (d) subtracting the midrange sensitivity from the lowpass        sensitivity to obtain a speaker-related sensitivity (as        indicated at step 625);    -   (e) performing an iterative search for current values of        parameters of an input-output model for the speaker using the        input signal spectrum, the stimulus signal spectrum, the        speaker-related sensitivity (as indicated at step 630); and    -   (f) updating averages of the parameters of the speaker        input-output model using the current parameter values (as        indicated at step 635).        The parameter averages of the speaker input-output model are        usable to perform echo cancellation on other input signals.

The input-output model of the speaker may be a nonlinear model, e.g., aVolterra series model.

Updating Modeling Information Based on Online Data Gathering

In one set of embodiments, the processor 207 may be programmed to updatethe modeling information I_(M) during periods of time when thespeakerphone 200 is being used to conduct a conversation.

Suppose speakerphone 200 is being used to conduct a conversation betweenone or more persons situated near the speakerphone 200 and one or moreother persons situated near a remote speakerphone (or videoconferencingsystem). In this case, the processor 207 sends out the remote audiosignal R(k), provided by the remote speakerphone, as the digital outputsignal Y(k). It would probably be offensive to the local persons if theprocessor 207 interrupted the conversation to inject a noisetransmission into the digital output stream Y(k) for the sake of selfcalibration. Thus, the processor 207 may perform its self calibrationbased on samples of the output signal Y(k) while it is “live”, i.e.,carrying the audio information provided by the remote speakerphone. Theself-calibration may be performed as follows.

The processor 207 may start storing samples of the output signal Y(k)into an first FIFO and storing samples of the input signal X(k) into asecond FIFO, e.g., FIFOs allocated in memory 209. Furthermore, theprocessor may scan the samples of the output signal Y(k) to determinewhen the average power of the output signal Y(k) exceeds (or at leastreaches) a certain power threshold. The processor 207 may terminate thestorage of the output samples Y(k) into the first FIFO in response tothis power condition being satisfied. However, the processor may delaythe termination of storage of the input samples X(k) into the secondFIFO to allow sufficient time for the capture of a full reverb tailcorresponding to the output signal Y(k) for a maximum expected roomsize.

The processor 207 may then operate, as described above, on a block B_(Y)of output samples stored in the first FIFO and a block B_(X) of inputsamples stored in the second FIFO to compute:

-   -   (1) current estimates for Volterra coefficients a_(i) and        b_(ij);    -   (2) a current estimate T_(mic) for the microphone transfer        function;    -   (3) updates for the average Volterra coefficients A_(i) and        B_(ij); and    -   (4) updates for the average microphone transfer function        H_(mic).        Because the block B_(X) of received input samples is captured        while the speakerphone 200 is being used to conduct a live        conversation, the block B_(X) is very likely to contain        interference (from the point of view of the self calibration)        due to the voices of persons in the environment of the        microphone 201. Thus, in updating the average values with the        respective current estimates, the processor may strongly weight        the past history contribution, i.e., more strongly than in those        situations described above where the self-calibration is        performed during periods of silence in the external environment.

In some embodiments, a system (e.g., a speakerphone or avideoconferencing system) may include a microphone, a speaker, memoryand a processor, e.g., as illustrated in FIG. 1B. The memory may beconfigured to store program instructions and data. The processor isconfigured to read and execute the program instructions from the memory.The program instructions are executable by the processor to:

-   -   (a) provide an output signal for transmission from the speaker,        where the output signal carries live signal information from a        remote source;    -   (b) receive an input signal from the microphone, corresponding        to the output signal and its reverb tail;    -   (c) compute a midrange sensitivity and a lowpass sensitivity for        a transfer function derived from a spectrum of the input signal        and a spectrum of the output signal;    -   (d) subtract the midrange sensitivity from the lowpass        sensitivity to obtain a speaker-related sensitivity;    -   (e) perform an iterative search for current values of parameters        of an input-output model for the speaker using the input signal        spectrum, the output signal spectrum, the speaker-related        sensitivity; and    -   (f) update averages of the parameters of the speaker        input-output model using the current values obtained in (e).        The parameter averages of the speaker input-output model are        usable to perform echo cancellation on other input signals        (i.e., other blocks of samples of the digital input signal        X(k)).

The input-output model of the speaker is a nonlinear model, e.g., aVolterra series model.

Furthermore, in some embodiments, the program instructions may beexecutable by the processor to:

-   -   perform an iterative search for a current transfer function of        the microphone using the input signal spectrum, the output        signal spectrum, and the current values; and    -   update an average microphone transfer function using the current        transfer function.        The current transfer function is usable to perform said echo        cancellation on said other input signals.

In one set of embodiments, as illustrated in FIG. 6B, a method forperforming self calibration may involve:

-   -   (a) providing an output signal for transmission from a speaker,        where the output signal carries live signal information from a        remote source (as indicated at step 660);    -   (b) receiving an input signal from a microphone, corresponding        to the output signal and its reverb tail (as indicated at step        665);    -   (c) computing a midrange sensitivity and a lowpass sensitivity        for a transfer function H(ω), where the transfer function H(ω)        is derived from a spectrum of the input signal and a spectrum of        the output signal (as indicated at step 670);    -   (d) subtracting the midrange sensitivity from the lowpass        sensitivity to obtain a speaker-related sensitivity (as        indicated at step 675);    -   (e) performing an iterative search for current values of        parameters of an input-output model for the speaker using the        input signal spectrum, the output signal spectrum and the        speaker-related sensitivity (as indicated at step 680); and    -   (f) updating averages of the parameters of the speaker        input-output model using the current parameter values (as        indicated at step 685).        The parameter averages of the speaker input-output model are        usable to perform echo cancellation on other input signals.

Furthermore, the method may involve:

-   -   performing an iterative search for a current transfer function        of the microphone using the input signal spectrum, the spectrum        of the output signal, and the current values; and    -   updating an average microphone transfer function using the        current transfer function.        The current transfer function is also usable to perform said        echo cancellation on said other input signals.        Plurality of Microphones

In some embodiments, the speakerphone 200 may include N_(M) inputchannels, where N_(M) is two or greater. Each input channel IC_(j), j=1,2, 3, . . . , N_(M) may include a microphone M_(j), a preamplifierPA_(j), and an A/D converter ADC_(j). The description given herein ofvarious embodiments in the context of one input channel naturallygeneralizes to N_(M) input channels.

Microphone M_(j) generates analog electrical signal u_(j)(t).Preamplifier PA_(j) amplifies the analog electrical signal u_(j)(t) inorder to generate amplified signal x_(j)(t). A/D converter ADC_(j)samples the amplified signal x_(j)(t) in order to generate digitalsignal X_(j)(k).

In one group of embodiments, the N_(M) microphones may be arranged in acircular array with the speaker 225 situated at the center of the circleas suggested by the physical realization (viewed from above) illustratedin FIG. 7. Thus, the delay time τ₀ of the direct path transmissionbetween the speaker and microphone M_(j) is approximately the same forall microphones. In one embodiment of this group, the microphones mayall be omni-directional microphones having approximately the samemicrophone transfer function.

Processor 207 may receive the digital input signals X_(j)(k), j=1, 2, .. . , N_(M), and perform acoustic echo cancellation on each channelindependently based on calibration information derived from each channelseparately.

In one embodiment, N_(M) equals 16. However, a wide variety of othervalues are contemplated for N_(M).

Direct Path Signal Cancellation Before AEC

In some embodiments, a speakerphone 300 may be configured as illustratedin FIG. 8. The reader will observe that speakerphone 300 is similar inmany respects to speakerphone 200 (illustrated in FIG. 1B). However, inaddition to the components illustrated in FIG. 1B as part ofspeakerphone 200, speakerphone 300 includes a subtraction circuit 310and a D/A converter 315. The subtraction circuit 310 is coupled toreceive:

-   -   the electrical signal u(t) generated by the microphone 201, and    -   the analog signal e(t) generated by the D/A converter 315.        The subtraction circuit 310 generates a difference signal        r(t)=u(t)−e(t). The difference signal r(t) is provided to        preamplifier circuit 203. Note that digital-to-analog (D/A)        converter 315 generates the signal e(t) from digital signal E(k)        and that the digital signal E(k) is provided by processor 207.

The preamplifier circuit 203 amplifies the difference signal r(t) togenerate an amplified signal x(t). The gain of the preamplifier circuitis adjustable within a specified dynamic range. Analog-to-digitalconverter 205 converts the amplified signal x(t) into a digital inputsignal X(k). The digital input signal X(k) is provided to processor 207.

The processor 207 receives a remote audio signal R(k) from anotherspeakerphone (e.g., via one or more of the communication interfaces 211)and mixes the remote audio signal R(k) with any locally generatedsignals (e.g., beeps or tones) to generate a digital output signal Y(k).

The digital-to-analog converter 240 receives the digital output signalY(k) and converts this signal into an analog electrical signal y(t). Thepower amplifier 250 amplifies the analog electrical signal y(t) togenerate an amplified signal v(t). The amplified signal v(t) is used todrive a speaker 225. The speaker 225 converts the amplified signal v(t)into an acoustic signal. The acoustic signal generated by the speakerradiates into the ambient space, and thus, local participants are ableto hear a replica of the acoustic signals generated by remoteparticipants (situated near a remote speakerphone).

FIG. 8B illustrates one embodiment of the speakerphone 300 whichincludes (among other things) an Ethernet bridge 211A, DDRAM 209A andFlash ROM 209B. The Ethernet bridge may couple to two connectors A andB.

In general, the microphone signal u(t) is a superposition ofcontributions due to:

-   -   acoustic signals (e.g., voice signals) generated by one or more        persons (e.g., conference participants) in the environment of        the speakerphone 300, and reflections of these acoustic signals        off of acoustically reflective surfaces in the environment;    -   acoustic signals generated by one or more noise sources (such as        fans and motors, automobile traffic and fluorescent light        fixtures) and reflections of these acoustic signals off of        acoustically reflective surfaces in the environment; and    -   the acoustic signal generated by the speaker 225 and the        reflections of this acoustic signal off of acoustically        reflective surfaces in the environment.        Let u_(dp)(t) denote the contribution to u(t) that corresponds        to the direct path transmission between speaker 225 and the        microphone 201. (See FIG. 2.)

Processor 207 may be configured to execute software including a directpath signal estimator 210 (hereinafter referred to as the DPS estimator)and an acoustic echo cancellation (AEC) module 220, e.g., as suggestedin FIG. 9. The DPS estimator and AEC module may be stored in memory 209.

The DPS estimator 210 may attempt to generate the digital signal E(k) sothat the corresponding analog signal e(t) is a good approximation to thedirect path contribution u_(dp)(t). In some embodiments, the DPSestimator may employ a method for generating digital signal E(k) thatguarantees (or approximates) the condition:Energy[e(t)−u _(dp)(t)]/Energy[u _(dp)(t)]<epsilon,where epsilon is a small positive fraction. The notation Energy[f(t)]represents the energy of the signal f(t) considered over a finiteinterval in time.

Because e(t) captures a substantial portion of the energy in the directpath contribution u_(dp)(t), the subtraction r(t)=u(t)−e(t) implies thatonly a small portion of the direct path contribution u_(dp)(t) remainsin r(t). The direct path contribution u_(dp)(t) is typically the mostdominant contribution to the microphone signal u(t). Thus, thesubtraction of e(t) from the microphone signal u(t) prior to thepreamplifier 203 implies that the average power in difference signalr(t) is substantially less than the average power in u(t). Therefore,the gain of the preamplifier may be substantially increased to moreeffectively utilize the dynamic range of the A/D converter 205 when theDPS estimator 210 is turned on. (When the DPS estimator is off, e(t)=0and r(t)=u(t).)

Note that the digital input signal X(k) is obtained from r(t) by scalingand sampling. Thus, it is apparent that the digital input signal X(k)would have a direct path contribution X_(dp)(k), linearly related tou_(dp)(t), if the DPS estimator 210 were turned off, i.e., if r(t)=u(t).However, only a small portion of the direct path contribution X_(dp)(k)remains in X(k) when the DPS estimator 210 is on, i.e., ifr(t)=u(t)−e(t). Any remaining portion of the direct path contributionX_(dp)(k) in digital input signal X(k) may fall below the threshold forconsideration by the AEC module 220. (In one embodiment, the AEC module220 may employ a threshold for deciding which peaks in the powerspectrum of X(k) are sufficiently large to warrant analysis.) Thus, theAEC module 220 will concentrate its computational effort on estimatingand canceling the reflected path contributions.

Because the AEC module 220 doesn't have to deal with the direct pathcontribution, the AEC module is able to analyze a larger number of thereflected path contributions than if it did have to deal with the directpath contribution. Furthermore, because the AEC module doesn't have todeal with the direct path contribution, the AEC module is able to setits dynamic range adjustment parameters in a manner that gives moreaccurate results in its analysis of the reflected path contributionsthan if the direct path signal estimator 210 were turned off. (If thedirect path estimator 210 were turned off, the direct path contributionX_(dp)(k) to the digital input X(k) would greatly dominate thecontributions due to the reflected paths.)

From the point-of-view of the AEC module 220, the path with minimumpropagation time (between speaker and microphone) is the first reflectedpath, i.e., the reflected path having the smallest path length, becausethe direct path is substantially eliminated from the digital input X(k).The propagation time τ₁ of the first reflected path is larger than thepropagation time τ₀ of the direct path. Thus, the AEC module 220 mayoperate on larger blocks of the samples X(k) than if the DPS estimator210 were turned off. The larger blocks of samples implies greaterfrequency resolution in the transform domain. Greater frequencyresolution implies a high-quality of cancellation of the reflectedpaths.

In various embodiments, the DPS estimator 210 receives signal Y(k) andoperates on the signal Y(k) using at least a subset of the modelinginformation I_(M) to generate the signal E(k). In one embodiment, theDPS estimator 210 may operate on the signal Y(k) using:

-   -   the gain of the D/A converter 240;    -   the gain of the power amplifier 250;    -   the input-output model for the speaker 225;    -   the transfer function H_(dp) for the direct path transmission        between the output of speaker 225 and the input of microphone        201;    -   the transfer function of the microphone 201;    -   the gain of the preamplifier 203; and    -   the gain of the A/D converter 205.

The DPS estimator 210 also receives the digital input X(k). Using blocksof the samples X(k) and corresponding blocks of the samples Y(k), theDPS estimator 210 may periodically update the transfer function H_(dp).For example, in some embodiments, the DPS estimator 210 may generate anew estimate of the transfer function H_(dp) for each received block ofdigital input X(k). The transfer function H_(dp) may be characterized byan attenuation coefficient and a time delay for the direct pathtransmission.

The AEC module 220 receives the digital input X(k) and the digitaloutput Y(k), generates an error signal C(k), and subtracts the errorsignal C(k) from the digital input X(k) to obtain a corrected signalZ(k). The corrected signal Z(k) may be transmitted to a remotespeakerphone through the communication mechanism CM. When the directpath signal estimator 210 is turned on, error signal C(k) generated bythe AEC module is an estimate of the portion of X(k) that is due to anumber N_(on) of the most dominant reflected path transmissions betweenthe speaker and the microphone. When the direct path signal estimator210 is turned off, the error signal C(k) generated by the AEC module isan estimate of the portion of X(k) that is due to the direct path and anumber N_(off) of the most dominant reflected path transmissions betweenthe speaker and the microphone. As alluded to above, when the DPSestimator 210 is on, the direct path contribution is substantiallyeliminated from the signal X(k) arriving at the AEC module 220 (byvirtue of the subtraction occurring at subtraction circuit 310). Thus,the AEC module 220 does not have to deal with the direct pathcontribution and is able to devote more of its computational resourcesto analyzing the reflected path contributions. Thus, N_(on) is generallylarger than N_(off).

The AEC module 220 may operate on the digital signal Y(k) using at leasta subset of the modeling information I_(M) in order to generate theerror signal C(k). In one embodiment, the AEC module 220 may operate onthe digital signal Y(k) using:

-   -   the gain of the D/A converter 240;    -   the gain of the power amplifier 250;    -   the apparent transfer function H_(app) between the output of        speaker 225 and the input of microphone 201;    -   the transfer function of the microphone 201;    -   the gain of the preamplifier 203;    -   the gain of the A/D converter 205.        Note that the apparent transfer function H_(app) models only        reflect paths between the speaker and microphone when the direct        path signal estimator 210 is turned on.

In some embodiments, a method for canceling speaker signal energy from areceived microphone signal may be enacted as illustrated in FIG. 9B.

At 930, samples of a digital output signal may be operated on todetermine samples of a digital correction signal. The output signalsamples are samples that are (or have been) directed to an outputchannel for transmission from a speaker.

At 932, the digital correction signal samples may be supplied to a firstdigital-to-analog converter for conversion into an analog correctionsignal.

At 934, a difference signal which is a difference between a first analogsignal provided by a microphone and the analog correction signal may begenerated (e.g., by an analog subtraction circuit), where the analogcorrection signal is an estimate of a contribution to the first analogsignal due to a direct path transmission between the speaker and themicrophone.

At 936, a digital input signal derived from the difference signal may bereceived from an input channel.

At 938, acoustic echo cancellation may be performed on the digital inputsignal to obtain a resultant signal. The acoustic echo cancellation maybe configured to remove contributions to the digital input signal due toreflected path transmissions between the speaker and the microphone.

Such a method may be especially useful for speakerphones andvideoconferencing system where a speaker and a microphone may be locatedclose to each other, e.g., on the housing of the speakerphone (orvideoconferencing system).

In one set of embodiments, the speakerphone 300 may include a set ofN_(M) input channels. Each input channel IC_(j), j=1, 2, 3, . . . ,N_(M), may include a microphone M_(j), a subtraction circuit SC_(j), apreamplifier PA_(j), an A/D converter ADC_(j), and a D/A converterDAC_(j). The integer N_(M) is greater than or equal to two. Thedescription given above of canceling the direct path contribution priorto the preamplifier 203 for one microphone channel naturally extends toN_(M) microphone channels. FIG. 10 illustrates speakerphone 300 in thecase N_(M)=16.

Let u_(j)(t) denote the analog electrical signal captured by microphoneM_(j). Subtraction circuit SC_(j) receives electrical signal u_(j)(t)and a corresponding correction signal e_(j)(t) and generates adifference signal r_(j)(t)=u_(j)(t)−e_(j)(t). Preamplifier PA_(j)amplifies the difference signal r_(j)(t) to obtain an amplified signalx_(j)(t). A/D converter ADC_(j) samples the amplified signal x_(j)(t) inorder to obtain a digital signal X_(j)(k). The digital signals X_(j)(k),j=1, 2, . . . , N_(M), are provided to processor 207.

Processor 207 generates the digital correction signals E_(j)(k), j=1, 2,. . . , N_(M). D/A converter DAC_(j) converts the digital correctionsignal E_(j)(k) into the analog correction signal e_(j)(t) which issupplied to the subtraction circuit SC_(j). Thus, the processor 207 maygenerate an independent correction signal E_(j)(k) for each inputchannel IC_(j) as described in the embodiments above.

In one group of embodiments, the N_(M) microphones may be arranged in acircular array with the speaker 225 situated at the center of thecircle, e.g., as suggested in FIG. 7. Thus, the delay time τ₀ of thedirect path transmission between the speaker and each microphone isapproximately the same for all microphones. Furthermore, the attenuationcoefficient of the direct path transmission between the speaker and eachmicrophone may be approximately the same for all microphones (since theyall have approximately the same distance from the center). Themicrophones may be configured to satisfy the condition of havingapproximately equal microphone transfer functions. This condition may beeasier to satisfy if the microphones are omnidirectional microphones. Insome embodiments, the processor 207 may apply the same correction signale(t) to each input channel, i.e., r_(j)(t)=u_(j)(t)−e(t) for j=1, 2, 3,. . . , N_(M). (FIG. 10B illustrates the case N_(M)=16.) In theseembodiments, the speakerphone 300 may have a D/A converter 315 which isshared among all input channels instead of N_(M) digital-to-analogconverters as described above. Thus, the processor 207 may generate asingle digital correction signal E(k) and supply the single correctionsignal E(k) to the D/A converter 315. The D/A converter 315 converts thecorrection signal E(k) into the analog correction signal e(t) which isfed to all the subtractions units SC_(j), j=1, 2, . . . , N_(M).

In one embodiment, N_(M) equals 16. However, a wide variety of othervalues are contemplated for N_(M).

In some embodiments, other microphone array configurations may be used(e.g., square, rectangular, elliptical, etc.).

In one set of embodiments, speakerphone 300 may be configured togenerate a correction signal E(k) from the digital output signal Y(k)by:

-   -   (a) multiplying the digital output signal Y(k) by the gain of        the D/A converter 240 and the gain of the power amplifier 250 to        obtain a digital representation v(k) of the speaker input        signal;    -   (b) applying a nonlinear speaker model to the digital        representation v(k) to obtain a digital representation R_(SP)(k)        of the acoustic signal radiated by the speaker 225;    -   (c) applying the transfer function H_(dp) (of the direct path        transmission from the speaker 225 to the microphone 201) to the        digital representation R_(SP)(k) to obtain a digital        representation A_(MIC)(k) of the acoustic signal received by the        microphone;    -   (d) applying the microphone transfer function to the digital        representation A_(MIC)(k) in order to obtain a digital        representation u(k) of the microphone output signal;    -   (e) multiplying the digital representation u(k) by the        reciprocal of the gain of the D/A converter 315.

Applying the transfer function H_(dp) to the digital representationR_(SP)(k) may involve:

-   -   delaying the digital representation R_(SP)(k) by the time delay        π₀ of the direct path transmission, and    -   scaling by the attenuation coefficient of the direct path        transmission.

The parameters of the nonlinear speaker model and the microphonetransfer function may change over time. Thus, the processor 207 mayrepeatedly update the model parameters and the microphone transferfunction in order to track the changes over time. Various embodimentsfor updating the speaker model parameters and the microphone transferfunction are described above.

Similarly, the speaker 225 and/or the microphone 201 may move, and thus,the transfer function H_(dp) may change over time. Thus, the processor207 may repeatedly update the transfer function H_(dp) as needed (e.g.,periodically or intermittently). The time delay τ₀ of the direct pathtransmission may be estimated based on a cross correlation between theoutput signal Y(k) and the input signal X(k). In one embodiment, theattenuation coefficient of the direct path transmission may be estimatedbased on a calibration experiment performed during a period of time whenthe speakerphone is not being used for communication and when theenvironment is relatively silent.

In one set of embodiments, the analog correction signal e(t) may besubtracted from raw signal u(t) coming from the microphone prior to thepreamplifier 203. In another set of embodiments, the analog correctionsignal may be subtracted after the preamplifier and prior to the A/Dconverter 205. In one alternative embodiment, the digital correctionsignal E(k) may be subtracted (in the digital domain) after the A/Dconverter 205 (and never converted into an analog signal).

In yet another set of embodiments, the analog correction signal e(t) maybe converted into an acoustic correction signal using a small acoustictransducer (e.g., speaker) situated close to the microphone 201. Thisacoustic cancellation methodology has the advantage of protecting themicrophone itself from clipping due to high volume sounds from thespeaker 225.

In some embodiments, the speakerphone 300 may have one or moremicrophones and one or more speakers arranged in a fixed configuration,e.g., mounted into the speakerphone housing. In other embodiments, theone or more microphones and one or more microphones may be movable,e.g., connected to the base unit by flexible wires and/or wirelessconnections. In yet other embodiments, some subset of the speakersand/or microphones may be fixed and another subset may be movable. Themethod embodiments described herein for canceling the direct pathcontribution to a microphone signal prior to preamplification (or priorto A/D conversion) may be applied to each microphone channel regardlessof whether the corresponding microphone is fixed or movable.

Cancellation of the direct path contribution from the raw microphonesignal u(t) may:

-   -   allow the usable dynamic range of the signal x(t) is be        increased by increasing the gain of the preamplifier 203;    -   reduce the closed loop gain of speaker-to-mic system;    -   improve echo canceller effectiveness by eliminating strong peaks        in the speaker-to-mic transfer function;    -   allow the speaker 225 to be driven at a louder volume and the        sensitivity of the microphone 201 to be increased without        clipping at the A/D converter 205, therefore allowing the        speakerphone 300 to function in larger rooms with larger        effective range because speaker 225 is louder and microphone 201        is more sensitive;    -   allow use of omnidirectional microphones instead of directional        microphones (such as cardioid or hypercardioid microphones).

Omnidirectional microphones are less expensive, more reliable and lesssensitive to vibration than directional microphones. Use of directionalmicrophones is complicated by the directional dependence of theirfrequency response. Omnidirectional microphones do not have thiscomplication. Omnidirectional microphones do not experience theproximity effect (this helps with dynamic range). Omnidirectionalmicrophones are smaller for the same sensitivity as directionalmicrophones, therefore allowing a smaller housing than if directionalmicrophones were used.

In one set of embodiments, the correction signal E(k) may be determinedas follows. The processor 207 may measure the transfer function H_(dp)of the direct path transmission between the speaker 225 and themicrophone 201, e.g., by asserting a noise burst as the output signalY(k) (for transmission from the speaker 225) and capturing the resultingsignal X(k) from the A/D converter 205. If this measurement is beingperformed in an environment having nontrivial echoes, the processor 207may reduce the duration of noise burst until the tail edge of the noiseburst arrives at the microphone 201 prior to the leading edge of thefirst room reflection. The processor 207 may assert the same noise burstrepeatedly in order to average out the effects of other random acousticsources in the room and the effects of circuit noise in the inputchannel (e.g., in the summation circuit 310, the preamplifier 203 andthe A/D converter 205).

The processor 207 may determine the minimum time interval betweensuccessive noise bursts based on the time it takes for the roomreverberation due to a single noise burst to die down to an acceptablylow level.

The processor 207 may perform a cross correlation between the noisestimulus Y(k) with measured response X(k) to determine the time delay τ₀between stimulus and response. In particular, the time delay τ₀ may bedetermined by the delay value which maximizes the cross correlationfunction.

In some embodiments, the precision of the measurement of time delay τ₀may be improved by adjusting the phase offset of the A/D converter 205and/or the phase offset of the D/A converter 240 relative to a baseconversion clock. The speakerphone 300 includes circuitry 410 to controlthe phase θ_(A/D) of the A/D conversion clock relative to the baseconversion clock and the phase θ_(D/A) of the D/A conversion clockrelative to the base conversion clock as suggested in FIG. 11. The A/Dconversion clock is supplied to the A/D converter 205 and controls whensampling events occur. The D/A conversion clock is supplied to the D/Aconverter 240 and controls when D/A conversion events occur. Thefrequency f_(conv) of the base conversion clock may be greater than orequal to the Nyquist rate for speech signals (or for audio signals insome embodiments). For example, in one embodiment the frequency f_(conv)may equal 16 kHz.

After having located the integer sample index k_(max) that maximizes thecross correlation, the processor 207 may:

-   -   (a) select a value of phase θ_(D/A);    -   (b) apply the selected phase value, e.g., by supplying the        selected phase value to the phase control circuitry 410;    -   (c) transmit the noise burst as the output signal Y(k);    -   (d) capture the response signal X(k) from the D/A converter 205;    -   (e) compute the cross correlation value (between the noise burst        and the response signal) corresponding to the integer sample        index k_(max);    -   (f) store the computed cross correlation value for further        analysis.

The processor 207 may repeat (a) through (f) for successive values ofphase θ_(D/A) spanning a range of angles, e.g., the range from −180 to180 degrees. Furthermore, the processor may analyze the successive crosscorrelation values to determine the value θ_(max) of the phase θ_(D/A)that gives the maximum cross correlation value. The processor 207 maycompute a refined estimate of the time delay τ₀ using the integer sampleindex k_(max) and the phase value θ_(max). For example, in oneembodiment, the processor 207 may compute the refined estimate accordingto the expression:τ₀ =k _(max)+θ_(max)/360.

In one set of embodiments, the processor 207 may increment the value ofphase θ_(D/A) by the angle (½^(N))*360 degrees, where N is a positiveinteger, in each iteration of (a). Thus, the processor 207 may explorethe phase valuesθ_(D/A)=−180+k*(½^(N))*360 degrees,k=0, 1, 2, . . . , 2^(N)−1. In one group of embodiments, N may equal anyinteger value in the range [3,9]. However, values outside this range arecontemplated as well.

In an alternative set of embodiments, the phase θ_(A/D) of the A/Dconverter 205 may be varied instead of the phase θ_(D/A) of the D/Aconverter 240.

In some embodiments, the processor 207 may compute:

a Fast Fourier Transform (FFT) of the noise burst that is transmitted asoutput Y(k);

an FFT of the response signal X(k) captured from the microphone inputchannel; and

a ratio H_(linear)=X(ω)/Y(ω), where Y(ω) denotes the transform of Y(k),and X(ω) denotes the transform of X(k). The ratio H_(linear)=X(ω)/Y(ω)represents the linear part of a model M describing the relationshipbetween signals at the circuit node corresponding to Y and the circuitnode corresponding to X. See FIG. 8.

In order to compute the parameters of the nonlinear part of the model M,the processor 207 may transmit sine wave tones (at two differentnon-harmonically related frequencies) as output Y(k), and, capture theresponse signal X(k) from the microphone input channel. The processormay compute the spectrum X(ω) of the response signal X(k) by performingan FFT, and equalize the spectrum X(ω) by multiplying the spectrum X(ω)by the inverse of the transfer function H_(linear) measured above:Y ^(eq)(ω)=X(ω)/H _(linear)(ω).The processor 207 may adapt the parameters of the nonlinear portionuntil the output of the model M closely matches the measured data.

In one set of embodiments, the model M may be a Volterra model.

During operation of the speakerphone 300, the processor 207 may transmitthe output signal Y(k) through the output channel (including D/Aconverter 240, power amplifier 250 and speaker 225) and capture theinput signal X(k) from the microphone input channel. Now the signal X(k)and Y(k) are carrying the substance of a live conversation between localparticipants and remote participants. The processor 207 may generate thecorrection signal E(k) by applying the non-linear portion of the model Mto the signal Y(k) in the time domain, and applying the linear portionof the model M to the spectrum Y(ω) in the frequency domain.

The parameters of the model M (including the linear portion and thenonlinear portion) may be recomputed periodically (or intermittently) inorder to track changes in the characteristics of the speaker andmicrophone. See the various embodiments described above for estimatingthe parameters of the model M.

The linear calibration may be performed during the night whenspeakerphone is less likely to be used and when people are less likelyto be in the room or near the room and when the air conditioning (or anyother noise sources that would reduce the accuracy of the measurement)is less likely to be operating. For example, the processor may beprogrammed to perform the calibration at 2:00 AM if a call is not inprogress and if the room is sufficiently quiet as determined by thesignal coming from the microphone(s).

Hybrid Beamforming

As noted above, speakerphone 300 (or speakerphone 200) may include a setof microphones, e.g., as suggested in FIG. 7. In one set of embodiments,processor 207 may operate on the set of digital input signals X_(j)(k),j=1, 2, . . . , N_(M), captured from the microphone input channels, togenerate a resultant signal D(k) that represents the output of a highlydirectional virtual microphone pointed in a target direction. Thevirtual microphone is configured to be much more sensitive in an angularneighborhood of the target direction than outside this angularneighborhood. The virtual microphone allows the speakerphone to “tunein” on any acoustic sources in the angular neighborhood and to “tuneout” (or suppress) acoustic sources outside the angular neighborhood.

According to one methodology, the processor 207 may generate theresultant signal D(k) by:

-   -   operating on the digital input signals X_(j)(k), j=1, 2, . . . ,        N_(M) with virtual beams B(1), B(2), . . . , B(N_(B)) to obtain        respective beam-formed signals, where N_(B) is greater than or        equal to two;    -   adding (perhaps with weighting) the beam-formed signals to        obtain a resultant signal D(k).        In one embodiment, this methodology may be implemented in the        frequency domain by:    -   computing a Fourier transform of the digital input signals        X_(j)(k), j=1, 2, . . . , N_(M), to generate corresponding input        spectra X_(j)(f), j=1, 2, . . . , N_(M), where f denotes        frequency; and    -   operating on the input spectra X_(j)(f), j=1, 2, . . . , N_(M)        with the virtual beams B(1), B(2), . . . , B(N_(B)) to obtain        respective beam formed spectra V(1), V(2), . . . , V(N_(B)),        where N_(B) is greater than or equal to two;    -   adding (perhaps with weighting) the spectra V(1), V(2), . . . ,        V(N_(B)) to obtain a resultant spectrum D(f);    -   inverse transforming the resultant spectrum D(f) to obtain the        resultant signal D(k).        Each of the virtual beams B(i), i=1, 2, . . . , N_(B) has an        associated frequency range        R(i)=[c _(i) ,d _(i)]        and operates on a corresponding subset S_(i) of the input        spectra X_(j)(f), j=1, 2, . . . , N_(M). (To say that A is a        subset of B does not exclude the possibility that subset A may        equal set B.) The processor 207 may window each of the spectra        of the subset S_(i) with a window function W_(i)(f)        corresponding to the frequency range R(i) to obtain windowed        spectra, and, operate on the windowed spectra with the beam B(i)        to obtain spectrum V(i). The window function W_(i) may equal one        inside the range R(i) and the value zero outside the range R(i).        Alternatively, the window function W_(i) may smoothly transition        to zero in neighborhoods of boundary frequencies c_(i) and        d_(i).

The union of the ranges R(1), R(2), . . . , R(N_(B)) may cover the rangeof audio frequencies, or, at least the range of frequencies occurring inspeech.

The ranges R(1), R(2), . . . , R(N_(B)) include a first subset of rangesthat are above a certain frequency f_(TR) and a second subset of rangesthat are below the frequency f_(TR). In one embodiment, the frequencyf_(TR) may be approximately 550 Hz.

Each of the virtual beams B(i) that corresponds to a frequency rangeR(i) below the frequency f_(TR) may be a superdirective beam of orderL(i) formed from L(i)+1 of the input spectra X_(j)(f), j=1, 2, . . . ,N_(M), where L(i) is an integer greater than or equal to one. The L(i)+1spectra may correspond to L(i)+1 microphones of the circular array thatare aligned (or approximately aligned) in the target direction.

Furthermore, each of the virtual beams B(i) that corresponds to afrequency range R(i) above the frequency f_(TR) may have the form of adelay-and-sum beam. The delay-and-sum parameters of the virtual beamB(i) may be designed by beam forming design software. The beam formingdesign software may be conventional software known to those skilled inthe art of beam forming. For example, the beam forming design softwaremay be software that is available as part of MATLAB®.

The beam forming design software may be directed to design an optimaldelay-and-sum beam for beam B(i) at some frequency f_(i) (e.g., themidpoint frequency) in the frequency range R(i) given the geometry ofthe circular array and beam constraints such as passband ripple δ_(P),stopband ripple δ_(S), passband edges θ_(P1) and θ_(P2), first stopbandedge θ_(S1) and second stopband edge θ_(S2) as suggested by FIG. 12.

The beams corresponding to frequency ranges above the frequency f_(TR)are referred to herein as “high-end beams”. The beams corresponding tofrequency ranges below the frequency f_(TR) are referred to herein as“low-end beams”. The virtual beams B(1), B(2), . . . , B(N_(B)) mayinclude one or more low-end beams and one or more high-end beams.

In some embodiments, the beam constraints may be the same for allhigh-end beams B(i). The passband edges θ_(P1) and θ_(P2) may beselected so as to define an angular sector of size 360/N_(M) degrees (orapproximately this size). The passband may be centered on the targetdirection θ_(T).

The high end frequency ranges R(i) may be an ordered succession ofranges that cover the frequencies from f_(TR) up to a certain maximumfrequency (e.g., the upper limit of audio frequencies, or, the upperlimit of voice frequencies).

The delay-and-sum parameters for each high-end beam and the parametersfor each low-end beam may be designed at a design facility and storedinto memory 209 prior to operation of the speakerphone.

Since the microphone array is symmetric with respect to rotation throughany multiple of 360/N_(M) degrees, in one set of embodiments, the set ofparameters designed for one target direction may be used for any of theN_(M) target directions given byk(360/N _(M)),k=0,1,2, . . . , N _(M)−1,by applying an appropriate circular shift when accessing the parametersfrom memory.

In one embodiment,

-   -   the frequency f_(TR) is 550 Hz,    -   R(1)=R(2)=[0.550 Hz],    -   L(1)=L(2)=2, and    -   low-end beam B(1) operates on three of the spectra X_(j)(f),        j=1, 2, . . . , N_(M), and low-end beam B(2) operates on a        different three of the spectra X_(j)(f), j=1, 2, . . . , N_(M);    -   frequency ranges R(3), R(4), . . . , R(N_(B)) are an ordered        succession of ranges covering the frequencies from f_(TR) up to        a certain maximum frequency (e.g., the upper limit of audio        frequencies, or, the upper limit of voice frequencies);    -   beams B(3), B(4), . . . , B(N_(M)) are high-end beams designed        as described above.        FIG. 13 illustrates the three microphones (and thus, the three        spectra) used by each of beams B(1) and B(2), relative to the        target direction.

In another embodiment, the virtual beams B(1), B(2), . . . , B(N_(B))may include a set of low-end beams of first order. FIG. 14 illustratesan example of three low-end beams of first order. Each of the threelow-end beams may be formed using a pair of the input spectra X_(j)(f),j=1, 2, . . . , N_(M). For example, beam B(1) may be formed from theinput spectra corresponding to the two “A” microphones. Beam B(2) may beformed form the input spectra corresponding to the two “B” microphones.Beam B(3) may be formed form the input spectra corresponding to the two“C” microphones.

In yet another embodiment, the virtual beams B(1), B(2), . . . ,B(N_(B)) may include a set of low-end beams of third order. FIG. 15illustrates an example of two low-end beams of third order. Each of thetwo low-end beams may be formed using a set of four input spectracorresponding to four consecutive microphone channels that areapproximately aligned in the target direction.

In one embodiment, the low order beams may include: second order beams(e.g., a pair of second order beams as suggested in FIG. 13), eachsecond order beam being associated with the range of frequencies lessthan f₁, where f₁ is less than f_(TR); and third order beams (e.g., apair of third order beams as suggested in FIG. 15), each third orderbeam being associated with the range of frequencies from f₁ to f_(TR).For example, f₁ may equal approximately 250 Hz.

In one set of embodiments, a method for generating a highly directedbeam may involve the following actions, as illustrated in FIG. 16A.

At 1605, input signals may be received from an array of microphones, oneinput signal from each of the microphones. The input signals may bedigitized and stored in an input buffer.

At 1610, low pass versions of at least a first subset of the inputsignals may be generated. Transition frequency f_(TR) may be the cutofffrequency for the low pass versions. The first subset of the inputsignals may correspond to a first subset of the microphones that are atleast partially aligned in a target direction. (See FIGS. 13-15 forvarious examples in the case of a circular array.)

At 1615, the low pass versions of the first subset of input signals areoperated on with a first set of parameters in order to compute a firstoutput signal corresponding to a first virtual beam having aninteger-order superdirective structure. The number of microphones in thefirst subset is one more than the integer order of the first virtualbeam.

At 1620, high pass versions of the input signals are generated. Again,the transition frequency f_(TR) may be the cutoff frequency for the highpass versions.

At 1625, the high pass versions are operated on with a second set ofparameters in order to compute a second output signal corresponding to asecond virtual beam having a delay-and-sum structure. The second set ofparameters may be configured so as to direct the second virtual beam inthe target direction.

The second set of parameters may be derived from a combination ofparameter sets corresponding to a number of band-specific virtual beams.For example, in one embodiment, the second set of parameters is derivedfrom a combination of the parameter sets corresponding to the high-endbeams of delay-and-sum form discussed above. Let N_(H) denote the numberof high-end beams. As discussed above, beam design software may beemployed to compute a set of parameters P(i) for a high-enddelay-and-sum beam B(i) at some frequency f_(i) in region R(i). The setP(i) may include N_(M) complex coefficients denoted P(i,j), j=1, 2, . .. , N_(M), i.e., one for each microphone. The second set Q of parametersmay be generated from the parameter sets P(i), i=1, 2, . . . , N_(H)according to the relation:

${{Q(j)} = {\sum\limits_{i = 1}^{N_{H}}{{P\left( {i,j} \right)}{U\left( {i,j} \right)}}}},$j=1, 2, . . . , N_(M), where U(i,j) is a weighting function that weightsthe parameters of set P(i), corresponding to frequency f_(i), mostheavily at microphone #i and successively less heavily at microphonesaway from microphone #i. Other schemes for combining the multipleparameter sets are also contemplated.

At 1630, a resultant signal is generated, where the resultant signalincludes a combination of at least the first output signal and thesecond output signal. The combination may be a linear combination orother type of combination. In one embodiment, the combination is astraight sum (with no weighting).

At 1635, the resultant signal may be provided to a communicationinterface for transmission to one or more remote destinations.

The action of generating low pass versions of at least a first subset ofthe input signals may include generating low pass versions of one ormore additional subsets of the input signals distinct from the firstsubset. Correspondingly, the method may further involve operating on theadditional subsets (of low pass versions) with corresponding additionalvirtual beams of integer-order superdirective structure. (There is norequirement that all the superdirective beams must have the same integerorder.) Thus, the combination (used to generate the resultant signal)also includes the output signals of the additional virtual beams.

The method may also involve accessing an array of parameters from amemory, and applying a circular shift to the array of parameters toobtain the second set of parameters, where an amount of the shiftcorresponds to the desired target direction.

It is noted that actions 1610 through 1630 may be performed in the timedomain, in the frequency domain, or partly in the time domain and partlyin the frequency domain. For example, 1610 may be implemented bytime-domain filtering or by windowing in the spectral domain. As anotherexample, 1625 may be performed by weighting, delaying and addingtime-domain functions, or, by weighting, adjusting and adding spectra.In light of the teachings given herein, one skilled in the art will notfail to understand how to implement each individual action in the timedomain or in the frequency domain.

In another set of embodiments, a method for generating a highly directedbeam may involve the following actions, as illustrated in FIG. 16B.

At 1640, input signals are received from an array of microphones, oneinput signal from each of the microphones.

At 1641, first versions of at least a first subset of the input signalsare generated, wherein the first versions are band limited to a firstfrequency range.

At 1642, the first versions of the first subset of input signals areoperated on with a first set of parameters in order to compute a firstoutput signal corresponding to a first virtual beam having aninteger-order superdirective structure.

At 1643, second versions of at least a second subset of the inputsignals are generated, wherein the second versions are band limited to asecond frequency range different from the first frequency range.

At 1644, the second versions of the second subset of input signals areoperated on with a second set of parameters in order to compute a secondoutput signal corresponding to a second virtual beam.

At 1645, a resultant signal is generated, wherein the resultant signalincludes a combination of at least the first output signal and thesecond output signal.

The second virtual beam may be a beam having a delay-and-sum structureor an integer order superdirective structure, e.g., with integer orderdifferent from the integer order of the first virtual beam.

The first subset of the input signals may correspond to a first subsetof the microphones which are at least partially aligned in a targetdirection. Furthermore, the second set of parameters may be configuredso as to direct the second virtual beam in the target direction.

Additional integer-order superdirective beams and/or delay-and-sum beamsmay be applied to corresponding subsets of band-limited versions of theinput signals, and the corresponding outputs (from the additional beams)may be combined into the resultant signal.

In another set of embodiments, a system may include a set ofmicrophones, a memory and a processor, e.g., as suggested variouslyabove in conjunction with FIGS. 1B, 7, 8, 8B, 10 and 10B. The memory maybe configured to store program instructions. The processor may beconfigured to read and execute the program instructions from the memory.The program instructions may be executable to implement:

-   -   (a) receiving input signals, one input signal corresponding to        each of the microphones;    -   (b) generating first versions of at least a first subset of the        input signals, wherein the first versions are band limited to a        first frequency range;    -   (c) operating on the first versions of the first subset of input        signals with a first set of parameters in order to compute a        first output signal corresponding to a first virtual beam having        an integer-order superdirective structure;    -   (d) generating second versions of at least a second subset of        the input signals, wherein the second versions are band limited        to a second frequency range different from the first frequency        range;    -   (e) operating on the second versions of the second subset of        input signals with a second set of parameters in order to        compute a second output signal corresponding to a second virtual        beam;    -   (f) generating a resultant signal, wherein the resultant signal        includes a combination of at least the first output signal and        the second output signal.        The second virtual beam may be a beam having a delay-and-sum        structure or an integer order superdirective structure, e.g.,        with integer order different from the integer order of the first        virtual beam.

The first subset of the input signals may correspond to a first subsetof the microphones which are at least partially aligned in a targetdirection. Furthermore, the second set of parameters may be configuredso as to direct the second virtual beam in the target direction.

Additional integer-order superdirective beams and/or delay-and-sum beamsmay be applied to corresponding subsets of band-limited versions of theinput signals, and the corresponding outputs (from the additional beams)may be combined into the resultant signal.

The program instructions may be further configured to direct theprocessor to provide the resultant signal to a communication interface(e.g., one of communication interfaces 211) for transmission to one ormore remote devices.

The set of microphones may be arranged on a circle. Other arraytopologies are contemplated. For example, the microphones may bearranged on an ellipse, a square, or a rectangle. In some embodiments,the microphones may be arranged on a grid, e.g., a rectangular grid, ahexagonal grid, etc.

In yet another set of embodiments, a method for generating a highlydirected beam may include the following actions, as illustrated in FIG.16C.

At 1650, input signals may be received from an array of microphones, oneinput signal from each of the microphones.

At 1655, the input signals may be operated on with a set of virtualbeams to obtain respective beam-formed signals, where each of thevirtual beams is associated with a corresponding frequency range and acorresponding subset of the input signals, where each of the virtualbeams operates on versions of the input signals of the correspondingsubset of input signals, where said versions are band limited to thecorresponding frequency range, where the virtual beams include one ormore virtual beams of a first type and one or more virtual beams of asecond type.

The first type and the second type may correspond to: differentmathematical expressions describing how the input signals are to becombined; different beam design methodologies; different theoreticalapproaches to beam forming, etc.

The one or more beams of the first type may be integer-ordersuperdirective beams. Furthermore, the one or more beams of the secondtype may be delay-and-sum beams.

At 1660, a resultant signal may be generated, where the resultant signalincludes a combination of the beam-formed signals.

The methods illustrated in FIGS. 16A-C may be implemented by one or moreprocessors under the control of program instructions, by dedicated(analog and/or digital) circuitry, or, by a combination of one or moreprocessors and dedicated circuitry. For example, any or all of thesemethods may be implemented by one or more processors in a speakerphone(e.g., speakerphone 200 or speakerphone 300).

In yet another set of embodiments, a method for configuring a targetsystem (i.e., a system including a processor, a memory and one or moreprocessors) may involve the following actions, as illustrated in FIG.17. The method may be implemented by executing program instructions on acomputer system which is coupled to the target system.

At 1710, a first set of parameters may be generated for a first virtualbeam based on a first subset of the microphones, where the first virtualbeam has an integer-order superdirective structure.

At 1715, a plurality of parameter sets may be computed for acorresponding plurality of delay-and-sum beams, where the parameter setfor each delay-and-sum beam is computed for a corresponding frequency,where the parameter sets for the delay-and-sum beams are computed basedon a common set of beam constraints. The frequencies for thedelay-and-sum beams may be above a transition frequency.

At 1720, the plurality of parameter sets may be combined to obtain asecond set of parameters, e.g., as described above.

At 1725, the first set of parameters and the second set of parametersmay be stored in the memory of the target system.

The delay-and-sum beams may be designed using beam forming designsoftware. Each of the delay-and-sum beams may be designed subject to thesame (or similar) set of beam constraints. For example, each of thedelay-and-sum beams may be constrained to have the same pass band width(i.e., main lobe width).

The target system being configured may be a device such as aspeakerphone, a videoconferencing system, a surveillance device, a videocamera, etc.

One measure of the quality of a virtual beam formed from a microphonearray is directivity index (DI). Directivity index indicates the amountof rejection of signal off axis from the desired signal. Virtual beamsformed from endfire microphone arrays (“endfire beams”) have anadvantage over beams formed from broadside arrays (“broadside beams”) inthat the endfire beams have constant DI over all frequencies as long asthe wavelength is greater than the microphone array spacing. (Broadsidebeams have increasingly lower DI at lower frequencies.) For endfirearrays, however, as the frequency goes down the signal level goes downby (6 dB per octave)×(endfire beam order) and therefore the gainrequired to maintain a flat response goes up, requiring highersignal-to-noise ratio to obtain a usable result.

A high DI at low frequencies is important because room reverberations,which people hear as “that hollow sound”, are predominantly at lowfrequencies. The higher the “order” of an endfire microphone array thehigher the potential DI value.

Calibration to Correct for Acoustic Shadowing

The performance of a speakerphone (such as speakerphone 200 orspeakerphone 300) using an array of microphones may be constrained by:

-   -   (1) the accuracy of knowledge of the 3 dimensional position of        each microphone in the array;    -   (2) the accuracy of knowledge of the magnitude and phase        response of each microphone;    -   (3) the signal-to-noise ratio (S/N) of the signal arriving at        each microphone; and    -   (4) the minimum acceptable signal-to-noise (S/N) ratio (as a        function of frequency) determined by the human auditory system.

(1) Prior to use of the speakerphone (e.g., during the manufacturingprocess), the position of each microphone in the speakerphone may bemeasured by placing the speakerphone in a test chamber. The test chamberincludes a set of speakers at known positions. The 3D position of eachmicrophone in the speakerphone may be determined by:

-   -   asserting a known signal from each speaker;    -   capturing the response from the microphone;    -   performing cross-correlations to determine the propagation time        of the known signal from each speaker to the microphone;    -   computing the propagation distance between each speaker and the        microphone from the corresponding propagation times;    -   computing the 3D position of the microphone from the propagation        distances and the known positions of the speakers.        It is noted that the phase of the A/D clock and/or the phase of        D/A clock may be adjusted as described above to obtain more        accurate estimates of the propagation times. The microphone        position data may be stored in non-volatile memory in each        speakerphone.

(2) There are two parts to having an accurate knowledge of the responseof the microphones in the array. The first part is an accuratemeasurement of the baseline response of each microphone in the arrayduring manufacture (or prior to distribution to customer). The firstpart is discussed below. The second part is adjusting the response ofeach microphone for variations that may occur over time as the productis used. The second part is discussed in detail above.

Especially at higher frequencies each microphone will have a differenttransfer function due to asymmetries in the speakerphone structure or inthe microphone pod. The response of each microphone in the speakerphonemay be measured as follows. The speakerphone is placed in a test chamberat a base position with a predetermined orientation. The test chamberincludes a movable speaker (or set of speakers at fixed positions). Thespeaker is placed at a first position in the test chamber. A calibrationcontroller asserts a noise burst through the speaker. The calibrationcontroller read and stores the signal X_(j)(k) captured by themicrophone M_(j), j=1, 2, . . . , N_(M), in the speakerphone in responseto the noise burst. The speaker is moved to a new position, and thenoise broadcast and data capture is repeated. The noise broadcast anddata capture are repeated for a set of speaker positions. For example,in one embodiment, the set of speaker positions may explore the circlein space given by:

-   -   radius equal to 5 feet relative to an origin at the center of        the microphone array;    -   azimuth angle in the range from zero to 360 degrees;    -   elevation angle equal to 15 degrees above the plane of the        microphone array.        In another embodiment, the set of speaker positions may explore        a region in space given by:    -   radius in the range form 1.5 feet to 20 feet.    -   azimuth angle in the range from zero to 360 degrees;    -   elevation angle in the range from zero to 90 degrees.        A wide variety of embodiments are contemplated for the region of        space sampled by the set of speaker positions.

A second speakerphone, having the same physical structure as the firstspeakerphone, is placed in the test chamber at the base position withthe predetermined orientation. The second speakerphone has idealmicrophones G_(j), j=1, 2, . . . , N_(M), mounted in the slots where thefirst speakerphone has less than ideal microphones M_(j). The idealmicrophones are “golden” microphones having flat frequency response. Thesame series of speaker positions are explored as with the firstspeakerphone. At each speaker position the same noise burst is assertedand the response X_(j) ^(G)(k) from each of the golden microphones ofthe second speakerphone is captured and stored.

For each microphone channel j and each speaker position, the calibrationcontroller may compute an estimate for the transfer function of themicrophone M_(j), j=1, 2, . . . , N_(M), according to the expression:H _(j) ^(mic)(ω)=X _(j)(ω)/X _(j) ^(G)(ω).The division by spectrum X_(j) ^(G)(ω) cancels the acoustic effects dueto the test chamber and the speakerphone structure. These microphonetransfer functions are stored into non-volatile memory of the firstspeakerphone, e.g., in memory 209.

In practice, it may be more efficient to gather the golden microphonedata from the second speakerphone first, and then, gather data from thefirst speakerphone, so that the microphone transfer functions H_(j)^(mic)(ω) for each microphone channel and each speaker position may beimmediately loaded into the first speakerphone before detaching thefirst speakerphone from the calibration controller.

In one embodiment, the first speakerphone may itself include software tocompute the microphone transfer functions H_(j) ^(mic)(ω) for eachmicrophone and each speaker position. In this case, the calibrationcontroller may download the golden response data to the firstspeakerphone so that the processor 207 of the speakerphone may computethe microphone transfer functions.

In some embodiments, the test chamber may include a platform that can berotated in the horizontal plane. The speakerphone may be placed on theplatform with the center of the microphone array coinciding with theaxis of the rotation of the platform. The platform may be rotatedinstead of attempting to change the azimuth angle of the speaker. Thus,the speaker may only require freedom of motion within a single planepassing through the axis of rotation of the platform.

When the speakerphone is being used to conduct a live conversation, theprocessor 207 may capture signals X_(j)(k) from the microphone inputchannels, j=1, 2, . . . , N_(M), and operate on the signals X_(j)(k)with one or more virtual beams as described above. The virtual beams arepointed in a target direction (or at a target position in space), e.g.,at an acoustic source such as a current talker. The beam design softwaremay have designed the virtual beams under the assumption that themicrophones are ideal omnidirectional microphones having flat spectralresponse. In order to compensate for the fact that the microphonesM_(j), j=1, 2, . . . , N_(M), are not ideal omnidirectional microphones,the processor 207 may access the microphone transfer functions H_(j)^(mic) corresponding to the target direction (or the target position inspace) and multiply the spectra X_(j)(ω) of the received signals by theinverses 1/H_(j) ^(mic)(ω) of the microphone transfer functionsrespectively:X _(j) ^(adj)(ω)=X _(j)(ω)/H _(j) ^(mic)(ω)The adjusted spectra X_(j) ^(adj)(ω) may then be supplied to the virtualbeam computations.

At high frequencies, effects such as acoustic shadowing begin to showup, in part due to the asymmetries in the speakerphone surfacestructure. For example, since the keypad is on one side of thespeakerphone's top surface, microphones near the keypad will experiencea different shadowing pattern than microphones more distant from thekeypad. In order to allow for the compensation of such effects, thefollowing calibration process may be performed. A golden microphone maybe positioned in the test chamber at a position and orientation thatwould be occupied by the microphone M₁ if the first speakerphone hadbeen placed in the test chamber. The golden microphone is positioned andoriented without being part of a speakerphone (because the intent is tocapture the acoustic response of just the test chamber.) The speaker ofthe test chamber is positioned at the first of the set of speakerpositions (i.e., the same set of positions used above to calibrate themicrophone transfer functions). The calibration controller asserts thenoise burst, reads the signal X₁ ^(C)(k) captured from microphone M₁ inresponse to the noise burst, and stores the signal X₁ ^(C)(k). The noiseburst and data capture is repeated for the golden microphone in each ofthe positions that would have been occupied if the first speakerphonehad been placed in the test chamber. Next, the speaker is moved to asecond of the set of speaker positions and the sequence ofnoise-burst-and-data-gathering over all microphone positions isperformed. The sequence of noise-burst-and-data-gathering over allmicrophone positions is performed for each of the speaker positions.After having explored all speaker positions, the calibration controllermay compute a shadowing transfer function H_(j) ^(SH)(ω) for eachmicrophone channel j=1, 2, . . . , N_(M), and for each speaker position,according to the expression:H _(j) ^(SH)(ω)=X _(j) ^(G)(ω)/X _(j) ^(C)(ω).The shadowing transfer functions may be stored in the memory ofspeakerphones prior to the distribution of the speakerphones tocustomers.

When a speakerphone is being used to conduct a live conversation, theprocessor 207 may capture signals X_(j)(k) from the microphone inputchannels, j=1, 2, . . . , N_(M), and operate on the signals X_(j)(k)with one or more virtual beams pointed in a target direction (or at atarget position) as described variously above. In order to compensatefor the fact that the microphones M_(j), j=1, 2, 3, . . . , N_(M), areacoustically shadowed (by being incorporated as part of a speakerphone),the processor 207 may access the shadow transfer functions H_(j)^(SH)(ω) corresponding to the target direction (or target position inspace) and multiply the spectra X_(j)(ω) of the received signals by theinverses 1/H_(j) ^(SH)(ω) of the shadowing transfer functionsrespectively:X _(j) ^(adj)(ω)=X _(j)(ω)/H _(j) ^(SH)(ω).The adjusted spectra X_(j) ^(adj)(ω) may then be supplied to the virtualbeam computations for the one or more virtual beams.

In some embodiments, the processor 207 may compensate for both non-idealmicrophones and acoustic shadowing by multiplying each received signalspectrum X_(j)(ω) by the inverse of the corresponding shadowing transferfunction for the target direction (or position) and the inverse of thecorresponding microphone transfer function for the target direction (orposition):

${X_{j}^{adj}(\omega)} = {\frac{X_{j}(\omega)}{{H_{j}^{SH}(\omega)}{H_{j}^{mic}(\omega)}}.}$The adjusted spectra X_(j) ^(adj)(ω) may then be supplied to the virtualbeam computations for the one or more virtual beams.

In some embodiments, parameters for a number of ideal high-end beams asdescribed above may be stored in a speakerphone. Each ideal high-endbeam B^(Id)(i) has an associated frequency range R_(i)=[c_(i),d_(i)] andmay have been designed (e.g., as described above, using beam designsoftware) assuming that: (a) the microphones are ideal omnidirectionalmicrophones and (b) there is no acoustic shadowing. The ideal beamB^(Id)(i) may be given by the expression:

${{{IdealBeamOutput}_{i}(\omega)} = {\sum\limits_{j = 1}^{N_{B}}{C_{j}{W_{i}(\omega)}{X_{j}(\omega)}{\exp\left( {{- {\mathbb{i}}}\;\omega\; d_{j}} \right)}}}},$where the attenuation coefficients C_(j) and the time delay values d_(j)are values given by the beam design software, and W_(i) is the spectralwindow function corresponding to frequency range R_(i). The failure ofassumption (a) may be compensated for by the speakerphone in real timeoperation as described above by multiplying by the inverses of themicrophone transfer functions corresponding to the target direction (ortarget position). The failure of the assumption (b) may be compensatedfor by the speakerphone in real time operation as described above byapplying the inverses of the shadowing transfer functions correspondingto the target direction (or target position). Thus, the corrected beamB(i) corresponding to ideal beam B^(Id)(i) may conform to theexpression:

${{CorrectedBeamOutput}_{i}(\omega)} = {\sum\limits_{j = 1}^{N_{B}}{C_{j}{W_{i}(\omega)}\frac{X_{j}(\omega)}{{H_{j}^{SH}(\omega)}{H_{j}^{mic}(\omega)}}{{\exp\left( {{- {\mathbb{i}}}\;\omega\; d_{j}} \right)}.}}}$In one embodiment, the complex value z_(i,j) of the shadowing transferfunction H_(j) ^(SH)(ω) at the center frequency (or some otherfrequency) of the range R_(i) may be used to simplify the aboveexpression to:

${{CorrectedBeamOutput}_{i}(\omega)} = {\sum\limits_{j = 1}^{N_{B}}{C_{j}{W_{i}(\omega)}\frac{X_{j}(\omega)}{H_{j}^{mic}(\omega)}{{\exp\left( {{- {\mathbb{i}}}\;\omega\; d_{j}} \right)}/{z_{i,j}.}}}}$A similar simplification may be achieved by replacing the microphonetransfer function H_(j) ^(mic)(ω) with its complex value at somefrequency in the range R_(i).

In one set of embodiments, a speakerphone may declare the failure of amicrophone in response to detecting a discontinuity in the microphonetransfer function as determined by a microphone calibration (e.g., anoffline self calibration or live self calibration as described above)and a comparison to past history information for the microphone.Similarly, the failure of a speaker may be declared in response todetecting a discontinuity in one or more parameters of the speakerinput-output model as determined by a speaker calibration (e.g., anoffline self calibration or live self calibration as described above)and a comparison to past history information for the speaker. Similarly,a failure in any of the circuitry interfacing to the microphone orspeaker may be detected.

At design time an analysis may be performed in order to predict thehighest order end-fire array achievable independent of S/N issues basedon the tolerances of the measured positions and microphone responses. Asthe order of an end-fire array is increased, its actual performancerequires higher and higher precision of microphone position andmicrophone response. By having very high precision measurements of thesefactors it is possible to use higher order arrays with higher DI thanpreviously achievable.

With a given maximum order array determined by tolerances, the requiredS/N of the system is considered, as that may also limit the maximumorder and therefore maximum usable DI at each frequency.

The S/N requirements at each frequency may be optimized relative to thehuman auditory system.

An optimized beam forming solution that gives maximum DI at eachfrequency subject to the S/N requirements and array tolerance of thesystem may be implemented. For example, consider an nht array with thefollowing formula:X=g1*mic1(t−d1)−g2*mic2(t−d2)− . . . gn*micn(t−dn).

Various mathematical solving techniques such an iterative solution or aKalman filter may be used to determine the required delays and gainsneeded to produce a solution optimized for S/N, response, tolerance, DIand the application.

For example, an array used to measure direction of arrival may need muchless S/N allowing higher DI than an application used in voicecommunications. There may be different S/N requirements depending on thetype of communication channel or compression algorithm applied to thedata.

Continuous Calibration Method

As seen in FIG. 18, a microphone 301 may have a diaphragm 303 (e.g., aMylar® diaphragm) in the form of a non-conductive membrane. One side ofthe membrane may be coated with a conductive coating. The other side ofthe membrane may be charged with a large positive charge at the time ofmanufacture. The charge may, however, slowly dissipate over the lifetimeof the microphone causing the microphone's response (i.e., transferfunction) to drift. Other microphone constructions are alsocontemplated. For example, in some embodiments, continuous calibrationmethods may be independent of the microphone construction and thereforework for microphones such as nanotype microphones, integrated circuitmicrophones, etc.

In some embodiments, a speakerphone may measure and compensate for drift(e.g., the speakerphone may measure changes in gain, phase, andfrequency response of microphones and correct for the drift). Forexample, a measurement of the signal from the microphone 301 (whichtypically includes a mixture of a dominant signal from a speaker andless dominant signals from other sources such as the voices ofparticipants in the room) may be stored. An average of the measurementsmay be taken over time. In some embodiments, the less dominant sourcesmay be insignificant in the time average compared to the dominantspeaker source. The time average may be compared to the speaker outputand the difference between the two may be used to offset the drift byadjusting the transfer function described above.

The amount of time used in the time average may depend on both the usagescenario and the microphone drift. In the case where there is a lot ofconstant background noise, the time averaging may be adjusted to belonger than in the case where the unit is in a quiet room. The driftwill vary between different microphones (even from the samemanufacturing lot) and will also vary depending on the environmentalconditions. For example, if the environment is constantly humid, theelectret element charge will dissipate more rapidly than in a dryenvironment. Average temperature will also affect the drift.

Various types of filters may be used. In some embodiments, the filteremploys a log type average (with the majority weighting on the “older”data). The transfer function may be calculated in real time and thenstored for “offline” processing along with a number of previously storeddata points. There may be a separate “microphone calibration” routinewhich is run when there are no other (more real-time) demands on theprocessor.

FIG. 19 illustrates a method for offsetting microphone drift, accordingto some embodiments. It should be noted that in various embodiments ofthe methods described below, one or more of the steps described may beperformed concurrently, in a different order than shown, or may beomitted entirely. Other additional steps may also be performed asdesired.

At 401, a signal from the speakerphone microphones may be measured. Insome embodiments, signals from each of the microphones may be measured.

At 403, microphone levels may be stored over time. In some embodiments,microphone levels from each microphone may be stored separately. In someembodiments, the microphone levels from each microphone may be addedtogether and the sum may be stored.

At 405, microphone levels may be averaged over time. In someembodiments, the microphone levels may be averaged after a predeterminedinterval of time. In some embodiments, the microphone levels may becontinuously averaged over time.

At 407, the time average of the microphone levels may be compared to aspeaker output level. For example, the speaker output may be subtractedfrom the microphone level time average for each microphone. If the timeaverage is for all of the microphone levels added together, the timeaverage may be divided by the number of speakers before the speakeroutput is subtracted out.

At 409, the transfer function discussed above with respect to thespeaker signal subtraction may be adjusted according to the differencebetween the time average of the microphone levels and the speakeroutput. For example, if there is a positive difference when the speakeroutput is subtracted from the time average, the positive difference maybe effectively subtracted from the microphone's response.

In addition to the frequency-domain transfer function discussed above,the center speaker signal (i.e., the signal generated by the centerspeaker of speakerphone systems such as speakerphone 200 in FIG. 7) maybe used in order to perform time-domain measurements. Such measurementsmay include tracking the variation of the total harmonic distortion ofthe speaker as a function of both input level and the average powerlevel. In this latter case, many speakers can exhibit short-termvariations in their output as a function of the temperature of the voicecoil. This phenomenon is not easily modeled as a linear system transferfunction and is typically referred to as “thermal compression”. Theseeffects may greatly influence the speaker output (and thus, thespeaker-to-microphone transfer function). Fortunately, they arerelatively easy to measure and do not typically change greatly overtime. However, if the speaker driver is damaged for some reason (if, forexample the unit is dropped from a large height onto a hard surface),then this damage might be easily detected, since thespeaker-to-microphone transfer function will thus change dramatically ina short period of time.

In various embodiments, another time-domain related measurement that canbe obtained from the system involves the exact relative positions of thespeaker and the microphone(s). This distance can be determined byexamining the acoustic delay between the speaker input signal and themicrophone output signal(s). Using a simple cross-correlation function,this delay can be calculated with reasonable accuracy typically withinone audio sample time, assuming that the acoustic path between thespeaker and the microphone(s) is not obstructed by some externalinterference. However, the reliability of such a cross-correlationmeasurement might be greatly increased if the speaker-to-microphonetransfer function is incorporated into the calculation. In addition, thetemporal resolution of such a cross-correlation measurement need not belimited to a single sample period. In order to increase the resolutionof such a calculation, the cross correlation can be conducted at a muchhigher effective sampling rate by constructing an interpolated datastream for both the speaker input and the microphone output signals.This interpolation prior to the cross-correlation measurement may beeffective in increasing the precision of the temporal delay resultobtained from the cross-correlation calculation. In the construction ofa virtual beam-formed output from the multiple real microphone outputs,this more exact knowledge of the true spatial locations of themicrophone array elements may provide a better result than a system thatdoes not take this information into account.

In various embodiments, two sets of variables may be maintained in themeasurement system; the microphone location(s) and the speaker location.The measurement may correct for relative movement between the speakerand microphone(s). However, if the speaker is oriented such that thatthe axis of its major motion is perpendicular to the direction of thecalibration measurement, the speaker may be much less likely to move inthe measured direction. The fact that the speaker is typically manytimes more massive than the microphone(s) and is also typically solidlyaffixed to a relatively inflexible structure (the speaker enclosure)also makes it much less likely to move than the relatively small andlight microphone(s). Thus, when calculating the relative positions ofthe microphone(s) and the speaker, the movement of the microphone(s)will typically dominate by an order of magnitude or more. In someembodiments, position estimation based on time-delay measurement may beperformed, and thus, the virtual beam-formed output may be adjusted toincrease its effectiveness. In some embodiments, the time-delaymeasurement may be made on a continuous or periodic basis. In order tocreate the “baseline” measurement for the system, a calibration sequencecan be conducted with an external speaker and microphone system that islocated in a known location in relation to the unit that is beingcalibrated. This absolute reference calibration may only need to beperformed once (at the time of manufacture) or it may also be performedin the case where the unit in question may be required to be serviced.In various embodiments, other methods of calibrating microphones may beused with the microphones in the speakerphone. In some embodiments, themethods may be used, for example, as a result of subtracting the speakeroutput and/or the difference between the speaker output and the timeaverage of the microphones.

In some embodiments, the system may self diagnose problems with variousspeakerphone components using drift calculations. For example, if thedrift is significant (e.g., greater than a pre-defined threshold), thesystem may determine that one or more speakerphone components aremalfunctioning. For example, the system may determine that the speakeror one or more microphones is damaged. The system may also determinewhether there is a problem with a component of the speakerphonecircuitry (e.g., a malfunctioning power amplifier). In some embodiments,the speakerphone may communicate the problem to a local user (e.g., bydisplaying or verbalizing an appropriate message). In some embodiments,the speakerphone may alert a user (or another individual or system) thatthere is a problem. For example, the speakerphone may send a messageover IP (e.g., using traps, email, SMS message, etc.).

Generalized Beam Forming

In various embodiments, given a uniform circular array 500 of physicalmicrophones as suggested by FIG. 20 and an arbitrary angle θ, any ofvarious transformations such as the Davies Transformation may be appliedto map the uniform circular array to a linear array 510 of virtualmicrophones oriented at angle θ with respect to a fixed ray which onecan think of as the positive x axis. The virtual microphones areillustrated as dashed circles.

The virtual linear array 510 may be used to estimate the direction ofarrival (DOA) of an acoustic signal generated by an acoustic source(e.g., a person's voice). It is a mathematical fact that the angularresolution of the DOA estimate from a linear array (physical or virtual)is highest when the DOA is normal to the axis of the linear array assuggested in FIG. 21A. (The axis of the linear array is the line alongwhich the microphones are placed.) A linear array that is oriented sothat the direction of arrival is normal to the array axis is said to bebroadside to the source.

There exist a number of well-known computational methods thatiteratively converge on a high-resolution DOA estimate by one or moreapplications of the Davies Transform (or some similar spatial frequencywarping method) to generate virtual linear arrays from a physicaluniform circular array. For example, these methods may involve startingwith a initial set of virtual linear arrays oriented at directionsspanning the circle, and then, iteratively converging on a linear arraythat is very close to being a broadside array.

In some embodiments, once a high resolution DOA estimate has beencomputed, the high-resolution DOA estimate may be used to construct anendfire array of virtual microphones (again using the Davies Transform).An endfire array is an array whose axis coincides with the direction ofarrival as suggested by FIG. 21B. The virtual endfire array may be usedto compute an estimate for the range (distance) of the acoustic source.Furthermore, the high resolution DOA estimate and the range estimate maybe used to construct an endfire array 520 of virtual microphones havingnon-uniform spacing (e.g., logarithmic spacing) from the uniformcircular array 500 as suggested in FIG. 21C. The range estimate may beused to optimally select the positions of the array elements.

A single super-directive virtual microphone 530 may be constructed fromthe logarithmic endfire array 520. The super-directive virtualmicrophone 530 has a sensitivity pattern which is highly directed towardthe acoustic source as suggested by FIG. 21D.

FIG. 21E illustrates a method for generating a highly directed virtualmicrophone pointed at an acoustic source using a uniform circular arrayof physical microphones. This method may be employed in a speakerphone,or, in any device having a uniform circular array of physicalmicrophones. In the case of a speakerphone, it may be used to generate avirtual microphone which is highly directed towards a current talker.

At 690, a processor (operating under the direction of programinstructions accessed from a storage medium) may compute ahigh-resolution estimate of the direction of arrival for an acousticsignal using virtual linear arrays constructed from a physical uniformcircular array. In one embodiment, one or more known algorithms may beemployed to perform this computation of the high-resolution DOAestimate. (The virtual linear arrays may be uniformly spaced arrays,i.e., arrays having uniform spacing between successive microphoneelements.)

At 692, the processor may generate a first virtual endfire array fromthe physical uniform circular array based on the direction of arrivalestimate. The first virtual endfire array may be a uniformly spacedarray.

At 694, the processor may compute a range estimate for the source of theacoustic signal using the first virtual endfire array.

At 696, the processor may generate a second virtual endfire array withnon-uniform spacing (e.g., with logarithmic spacing) from the physicaluniform circular array based on the direction of arrival estimate andthe range estimate.

At 698, the processor may generate a single virtual microphone which ishighly directed at the acoustic source from the second virtual endfirearray using the range estimate.

This method may be repeated (e.g., on a periodic basis) to track amoving source. It is noted that once a source is initially located,successive repetitions of 690 may be performed much more quickly thanthe initial DOA estimate since the DOA algorithm can immediately startwith a virtual linear array that is close to being broadside to thesource (under the assumption that the source typically has changed muchin angle in the time between repetitions).

High Resolution Distance Estimation for an Unknown Source

In some embodiments, when calculating a value for the range of anarbitrary source (i.e., the distance from an unknown source to thereceiving microphone array), we use the fact that the Direction ofArrival (DOA) of a signal that is propagating in a substantially similardirection as the major axis of the endfire array can be determined withsome accuracy. If we combine the DOA estimates for two such arrays thatare aligned in slightly different directions, then we can use theintersection of these two DOA estimates to determine the distance of thesource from the receiving array with reasonably good accuracy. If wecombine a single, highly accurate DOA estimation (such as that which wecould get from a broadside array) with a slightly less accurate DOAestimation (such as that which we could obtain from an endfire arraywhich is nearly in line with the source), then we can get a highlyaccurate estimate of the distance from the source to the two arrays. If,however, the source is in the nearfield for some frequencies and in thefar field for other frequencies, then we can use this information to getan accurate estimate for the range of the target at differentfrequencies and thus, the accuracy of the distance of the source is veryhighly accurate, since the equations for DOA estimation for thenearfield and the farfield case are different.

In some embodiments, a method for correcting for imperfections inmicrophones of a microphone array in a device such as a speakerphone (orvideoconferencing unit) may involve: (a) measuring responses of arraymicrophones to a noise burst for different speaker positions; (b)measuring responses of embedded golden microphones to the noise burstfor the same speaker positions, where the embedded golden microphonesare embedded in a second speakerphone; (c) computing microphone transferfunctions for each speaker position from the responses measured in (a)and (b); storing the microphone transfer functions in memory of thedevice for later use to correct received signals from the microphonearray.

In some embodiments, a method for correcting for acoustic shadowing ofmicrophones in a microphone array in a device such as a speakerphone (orvideoconferencing unit) may involve: (a) measuring responses of goldenmicrophones to a noise burst for different speaker positions, where thegolden microphones are embedded in a speakerphone; (b) measuringresponses of free golden microphones to the noise burst for the samespeaker positions; computing shadowing transfer functions for eachspeaker position from the responses measured in (a) and (b); adjustingthe parameters of a virtual beam corresponding to a first frequencyrange and a first target direction using a values of the shadowingtransfer function corresponding to the first frequency range and thefirst target direction.

In some embodiments, a method for tracking the drift in the response ofa microphone in a speakerphone may involve: (a) measuring a signal fromthe microphone; (b) storing a plurality of signal measurements from themicrophone; (b) averaging at least a portion of the stored plurality ofsignal measurements over time; (c) subtracting a speaker output from theaveraged signal measurement; and (d) adjusting a transfer function(e.g., a transfer function of the microphone) using the differencebetween the speaker output and the averaged signal measurement.

In some embodiments, a method of forming a highly directive virtualmicrophone from a circular array of microphones in a device (such as aspeakerphone or a videoconferencing unit) may involve: determining adirection of arrival of a source signal from analysis of signalsgathered from the microphones of the circular array; generating a firstvirtual endfire array pointed at the acoustic source using the directionof arrival; estimating distance to the source from signals provided bythe first virtual endfire array; generating a second virtual endfirearray that is nonuniformly spaced and pointed at the acoustic sourceusing the range estimate and the direction of arrival; combining signalsfrom the second virtual endfire array to obtain a resultant signalcorresponding to a highly directive virtual microphone.

Microphone/Speaker Calibration Processes

A stimulus signal may be transmitted by the speaker. The returned signal(i.e., the signal sensed by the microphone array) may be used to performcalibration. This returned signal may include four basic signalcategories (arranged in order of decreasing signal strength as seen bythe microphone):

1) internal audio

-   -   a: structure-borne vibration and/or radiated audio    -   b: structure-generated audio (i.e., buzzes and rattles)

2) first arrival (i.e., direct air-path) radiated audio

3) room-related audio

-   -   a: reflections    -   b: resonances

4) measurement noise

-   -   a: microphone self-noise    -   b: external room noise

Each of these four categories can be further broken down into separateconstituents. In some embodiments, the second category is measured inorder to determine the microphone calibration (and microphone changes).

Measuring Internal Audio

In one set of embodiment, one may start by measuring the first type ofresponse at the factory in a calibration chamber (where audio signals oftype 3 or 4 do not exist) and subtracting that response from subsequentmeasurements. By comparison with a “golden unit”, one knows how audio oftype 1 a) should measure, and one can then measure microphone self-noise(type 4 b) by recording data in a silent test chamber, so one canseparate the different responses listed above by making a small set ofsimple measurements in the factory calibration chamber.

It is noted that a “failure” caused by 1 b) may dominate themeasurements. Furthermore, “failures” caused by 1 b) may changedramatically over time, if something happens to the physical structure(e.g., if someone drops the unit or if it is damaged in shipping or ifit is not well-assembled and something in the internal structure shiftsas a result of normal handling and/or operation).

Fortunately, in a well-put together unit, the buzzes and rattles areusually only excited by a limited band of frequencies (e.g., those wherethe structure has a natural set of resonances). One can previouslydetermine these “dangerous frequencies” by experiment and by measuringthe “golden unit(s)”. One removes these signals from the stimulus beforemaking the measurement by means of a very sharp notch in the frequencyresponse of signals that are transmitted to the speaker amp.

In one embodiment, these frequencies may be determined by running asmall amplitude swept-sine stimulus through the unit's speaker andmeasure the harmonic distortion of the resulting raw signal that showsup in the microphones. In the calibration chamber, one can measure thedistortion of the speaker itself (using an external referencemicrophone) so one can know even the smallest levels of distortioncaused by the speaker as a reference. If the swept sine is kept smallenough, then one knows a-priori that the loudspeaker should nottypically be the major contributor to the distortion.

If the calibration procedure is repeated in the field, and if there isdistortion showing up at the microphones, and if it is equal over all ofthe microphones, then one knows that the loudspeaker has been damaged.If the microphone signals show non-equal distortion, then one may beconfident that it is something else (typically an internal mechanicalproblem) that is causing this distortion. Since the speaker may be theonly internal element which is equidistant from all microphones, one candetermine if there is something else mechanical that is causing thedistortions by examining the relative level (and phase delay, in somecases) of the distortion components that show up in each of the rawmicrophone signals.

So, one can analyze the distortion versus frequency for all of themicrophones separately and determine where the buzzing and/or rattlingcomponent is located and then use this information to make manufacturingimprovements. For example, one can determine, through analysis of theraw data, whether a plastic piece that is located between microphones 3and 4 is not properly glued in before the unit leaves the factory floor.As another example, one can also determine if a screw is coming looseover time. Due to the differences in the measured distortion and/orfrequency response seen at each of the mics, one can also determine thedifference between one of the above failures and one that is caused by amic wire that has come loose from its captive mounting, since theanomalies caused by that problem have a very different characteristicthan the others.

Measurement Noise

One can determine the baseline microphone self-noise in a factorycalibration chamber. In the field, however, it may be difficult toseparate out the measurement of the microphone's self-noise and the roomnoise unless one does a lot of averaging. Even then, if the room noiseis constant (in amplitude), one cannot completely remove it from themeasurement. However, one can wait for the point where the overall noiselevel is at a minimum (for example if the unit wakes up at 2:30 am and“listens” to see if there is anyone in the room or if the HVAC fan ison, etc.) and then minimize the amount of room noise that one will seein the overall microphone self noise measurement.

Another strategy is if the room has anisotropic noise (i.e., if thenoise in the room has some directional characteristic). Then one canperform beam-forming on the mic array, find the direction that the noiseis strongest, measure its amplitude and then measure the noise soundfield (i.e., its spatial characteristic) and then use that to come upwith an estimate of how large a contribution that the noise field willmake at each microphone's location. One then subtracts that value fromthe measured microphone noise level in order to separate the room noisefrom the self-noise of the mic itself.

Room-Related Audio Measurement

There are two components of the signal seen at each mic that are due tothe interactions of the speaker stimulus signal and the room in whichthe speaker is located: reflections and resonances. One can use the micarray to determine the approximate dimensions of the room by sending astimulus out of the loudspeaker and then measuring the first time ofreflection from all directions. That will effectively tell one where thewalls and ceiling are in relation to the speakerphone. From thisinformation, one can effectively remove the contribution of thereflections to the calibration procedure by “gating” the dataacquisition from the measured data sets from each of the mics. Thisgating process means that one only looks at the measured data duringspecific time intervals (when one knows that there has not been enoughtime for a reflection to have occurred).

The second form of room related audio measurement may be factored in aswell. Room-geometry related resonances are peaks and nulls in thefrequency response as measured at the microphone caused by positive andnegative interference of audio waveforms due to physical objects in theroom and due to the room dimensions themselves. Since one is gating themeasurement based on the room dimensions, then one can get rid of thelatter of the two (so-called standing waves). However, one may stillneed to factor out the resonances that are caused by objects in the roomthat are closer to the phone than the walls (for example, if the phoneis sitting on a wooden table that resonates at certain frequencies). Onecan deal with these issues much in the same way that one deals with theproblematic frequencies in the structure of the phone itself; by addingsharp notches in the stimulus signal such that these resonances are notexcited. The goal is to differentiate between these kinds of resonancesand similar resonances that occur in the structure of the phone itself.Three methods for doing this are as follows: 1) one knows a-priori wherethese resonances typically occur in the phone itself, 2) externalresonances tend to be lower in frequency than internal resonances and 3)one knows that these external object related resonances only occur aftera certain time (i.e., if one measures the resonance effects at theearliest time of arrival of the stimulus signal, then it will bedifferent than the resonance behavior after the signal has had time toreflect off of the external resonator).

So, after one factors in all of the adjustments described above, onethen can isolate the first arrival (i.e., direct air-path) radiatedaudio signal from the rest of the contributions to the mic signal. Thatis how one can perform accurate offline (and potentially online) mic andspeaker calibration.

Any or all of the method embodiments described herein may be implementedin terms of program instructions (executable by one or more processors)and stored on a memory medium. A memory medium may include any ofvarious types of memory devices or storage devices. The term “memorymedium” is intended to include an installation medium, e.g., a CD-ROM,floppy disks, or tape device; a computer system memory or random accessmemory such as DRAM, DDR RAM, SRAM, EDO RAM, Rambus RAM, etc.; or anon-volatile memory such as a magnetic media, e.g., a hard drive, oroptical storage. The memory medium may comprise other types of memory aswell, or combinations thereof. In addition, the memory medium may belocated in a first computer in which the programs are executed, or maybe located in a second different computer that connects to the firstcomputer over a network, such as the Internet. In the latter instance,the second computer may provide program instructions to the firstcomputer for execution. The term “memory medium” may include two or morememory mediums that may reside in different locations, e.g., indifferent computers that are connected over a network. In someembodiments, a carrier medium may be used. A carrier medium may includea memory medium as described above, as well as signals such aselectrical, electromagnetic, or digital signals, conveyed via acommunication medium such as a bus, network and/or a wireless link.

The memory medium may comprise an electrically erasable programmableread-only memory (EEPROM), various types of flash memory, etc. whichstore software programs (e.g., firmware) that are executable to performthe methods described herein. In some embodiments, field programmablegate arrays may be used. Various embodiments further include receivingor storing instructions and/or data implemented in accordance with theforegoing description upon a carrier medium.

CONCLUSION

Various embodiments may further include receiving, sending or storingprogram instructions and/or data implemented in accordance with theforegoing description upon a computer-accessible medium. Generallyspeaking, a computer-accessible medium may include storage media ormemory media such as magnetic or optical media, e.g., disk or CD-ROM,volatile or non-volatile media such as RAM (e.g. SDRAM, DDR SDRAM,RDRAM, SRAM, etc.), ROM, etc. as well as transmission media or signalssuch as electrical, electromagnetic, or digital signals, conveyed via acommunication medium such as network and/or a wireless link.

The various methods as illustrated in the Figures and described hereinrepresent exemplary embodiments of methods. The methods may beimplemented in software, hardware, or a combination thereof. The orderof method may be changed, and various elements may be added, reordered,combined, omitted, modified, etc.

Various modifications and changes may be made as would be obvious to aperson skilled in the art having the benefit of this disclosure. It isintended that the invention embrace all such modifications and changesand, accordingly, the above description to be regarded in anillustrative rather than a restrictive sense.

1. A system comprising: a set of microphones; a memory that storesprogram instructions; a processor configured to read and execute theprogram instructions from the memory, wherein the program instructionsare executable to implement: (a) receiving input signals, wherein eachof the input signals is received from a corresponding one of themicrophones; (b) operating on a first subset of the input signals torespectively generate first versions of the input signals in the firstsubset, wherein the first versions are band limited to a first frequencyrange, wherein the first frequency range is below a transitionfrequency; (c) operating on the first versions of the input signals inthe first subset with a first set of beam parameters in order to computea first output signal, wherein the first set of beam parameterscorresponds to a first virtual beam having an integer-ordersuperdirective structure; (d) operating on a second subset of the inputsignals to respectively generate second versions of the input signals inthe second subset, wherein the second versions are band limited to asecond frequency range different from the first frequency range, whereinthe second frequency range is above the transition frequency, whereinthe second subset includes two or more of the input signals and is notidentical to the first subset of the input signals; (e) operating on thesecond versions of the input signals in the second subset with a secondset of beam parameters in order to compute a second output signal,wherein the second set of beam parameters corresponds to a secondvirtual beam having a delay-and-sum structure; (f) generating aresultant signal, wherein the resultant signal includes a combination ofat least the first output signal and the second output signal.
 2. Thesystem of claim 1, wherein the first subset of the input signalscorresponds to a first subset of the microphones, wherein the firstsubset of microphones have physical positions that are at leastpartially aligned in a target direction, wherein the second set of beamparameters are configured so as to direct the second virtual beam in thetarget direction.
 3. The system of claim 1, wherein (b) through (e) areperformed in the frequency domain.
 4. The system of claim 1, wherein theprogram instructions are further executable by the processor to providethe resultant signal to a communication interface for transmission. 5.The system of claim 1, wherein the set of microphones are arranged in acircular array.
 6. A method comprising: receiving input signals from anarray of microphones, wherein each of the input signals in received froma corresponding one of the microphones; operating on the input signalswith a set of virtual beams to obtain respective beam-formed signals,wherein each of the virtual beams is associated with a correspondingfrequency range and a corresponding subset of the input signals, whereineach of the virtual beams operates on correspondingly band-limitedversions of the input signals of the corresponding subset, wherein saidband-limited versions operated on by each virtual beam are band limitedto the corresponding frequency range, wherein the virtual beams includeone or more integer-order superdirective beams, wherein the virtualbeams also include one or more delay-and-sum beams, wherein the one ormore frequencies ranges that correspond to the one or more integer-ordersuperdirective beams are below a transition frequency, wherein the oneor more frequencies ranges that correspond to the one or moredelay-and-sum beams are above the transition frequency; and generating aresultant signal, wherein the resultant signal includes a combination ofthe beam-formed signals; wherein said receiving, said operating and saidgenerating are performed by a computer system.
 7. The method of claim 6,wherein a first of the one or more integer-order superdirective beams isassociated with a first frequency range and a first subset of the inputsignals, wherein a first of the one or more delay-and-sum beams isassociated with a second frequency range and a second subset of theinput signals, wherein the first subset is not identical to the secondsubset, wherein the method further comprises: operating on the firstsubset of the input signals to respectively generate first band-limitedversions of the input signals of the first subset, wherein the firstband-limited versions are band-limited to the first frequency range;operating on the second subset of the input signals to respectivelygenerate second band-limited versions of the input signals of the secondsubset, wherein the second band-limited versions are band-limited to thesecond frequency range; wherein said operating on the input signals witha set of virtual beams to obtain respective beam-formed signalscomprises: operating on the first band-limited versions of the inputsignals in the first subset with a first set of beam parameters in orderto compute a first beam-formed signal corresponding to the firstinteger-order superdirective beam; operating on the second band-limitedversions of the input signals in the second subset with a second set ofbeam parameters in order to compute a second beam-formed signalcorresponding to the first delay-and-sum beam.
 8. The method of claim 7,wherein the first subset of the input signals corresponds to a firstsubset of the microphones, wherein the first subset of microphones havespatial positions that are at least partially aligned in a targetdirection, wherein the second set of beam parameters are configured soas to direct the first delay-and-sum beam in the target direction. 9.The method of claim 7, wherein said operating on the input signals witha set of virtual beams to obtain respective beam-formed signals isperformed in the frequency domain.
 10. The method of claim 7 furthercomprising: providing the resultant signal to a communication interfacefor transmission.
 11. The method of claim 7, wherein the array ofmicrophones are arranged on a circle.
 12. The method of claim 11 furthercomprising: accessing an array of parameters from a memory; applying acircular shift to the array of parameters to obtain the second set ofbeam parameters, wherein the circular shift is with respect tomicrophone position index, wherein an amount of the shift corresponds toa desired target direction.
 13. A non-transitory computer-readablememory medium that stores program instructions, wherein the programinstructions are executable by a computer system to implement: (a)receiving input signals from an array of microphones, wherein each ofthe input signals is received from a corresponding one of themicrophones; (b) operating on a first subset of the input signals torespectively generate first versions of the input signals in the firstsubset, wherein the first versions are band-limited to a first frequencyrange, wherein the first frequency range is below a transitionfrequency; (c) operating on the first versions of the first subset ofinput signals with a first set of beam parameters in order to compute afirst output signal, wherein the first set of beam parameterscorresponds to a first virtual beam having an integer-ordersuperdirective structure; (d) operating on a second subset of the inputsignals to respectively generate second versions of the input signals inthe second subset, wherein the second versions are band limited to asecond frequency range different from the first frequency range, whereinthe second frequency range is above the transition frequency, whereinthe second subset of the input signals is not identical to the firstsubset of the input signals; (e) operating on the second versions of theinput signals in the second subset with a second set of beam parametersin order to compute a second output signal, wherein the second set ofbeam parameters corresponds to a second virtual beam having adelay-and-sum structure; (f) generating a resultant signal, wherein theresultant signal includes a combination of at least the first outputsignal and the second output signal.
 14. A method for configuring adevice that includes a computer system, wherein the device includes anarray of microphones, a memory and a processor, the method comprising:(a) generating a first set of beam parameters for a first virtual beampointed in a target direction, wherein said generating is based on afirst subset of the microphones, wherein the first virtual beam has aninteger-order superdirective structure, wherein the first set of beamparameters are generated for a first frequency band; (b) computing aplurality of parameter sets for a corresponding plurality ofdelay-and-sum beams pointed in the target direction, wherein theparameter set for each delay-and-sum beam is computed for acorresponding frequency, wherein the parameter sets for thedelay-and-sum beams are computed based on a common set of beamconstraints and based on a geometric description of the microphonearray, wherein the corresponding frequencies for the delay-and-sum beamsare above a transition frequency, wherein the first frequency band isentirely below the transition frequency; (c) combining the plurality ofparameter sets to obtain a second set of beam parameters; and (d)storing the first set of beam parameters and the second set of beamparameters in the memory of the computer system; wherein (a), (b), (c)and (d) are performed by the computer system.
 15. The method of claim14, wherein the device is a speakerphone.
 16. The method of claim 14,wherein the common set of beam constraints includes a pass band width.17. The system of claim 1, wherein the second subset of the inputsignals includes all the input signals.
 18. The method of claim 6,wherein each subset that corresponds to one of the delay-and-sum beamsincludes all of the input signals.
 19. The system of claim 1, whereinthe first subset of the input signals includes two or more of the inputsignals but less than all the input signals.